CHAPTER 3

The Structure of the Java Virtual Machine

This book specifies an abstract machine. It does not document any particular implementation of the Java virtual machine, including Sun Microsystems'.

To implement the Java virtual machine correctly, you need only be able to read the class file format and correctly perform the operations specified therein. Implementation details that are not part of the Java virtual machine's specification would unnecessarily constrain the creativity of implementors. For example, the memory layout of run-time data areas, the garbage-collection algorithm used, and any internal optimization of the Java virtual machine instructions (for example, translating them into machine code) are left to the discretion of the implementor.

3.1 The class File Format

Chapter 4, "The class File Format", covers the class file format in detail.

3.2 Data Types

The Java virtual machine expects that nearly all type checking is done prior to run time, typically by a compiler, and does not have to be done by the Java virtual machine itself. Values of primitive types need not be tagged or otherwise be inspectable to determine their types at run time, or to be distinguished from values of reference types. Instead, the instruction set of the Java virtual machine distinguishes its operand types using instructions intended to operate on values of specific types. For instance, iadd, ladd, fadd, and dadd are all Java virtual machine instructions that add two numeric values and produce numeric results, but each is specialized for its operand type: int, long, float, and double, respectively. For a summary of type support in the Java virtual machine instruction set, see §3.11.1.

The Java virtual machine contains explicit support for objects. An object is either a dynamically allocated class instance or an array. A reference to an object is considered to have Java virtual machine type reference. Values of type reference can be thought of as pointers to objects. More than one reference to an object may exist. Objects are always operated on, passed, and tested via values of type reference.

3.3 Primitive Types and Values

• byte, whose values are 8-bit signed two's-complement integers

• short, whose values are 16-bit signed two's-complement integers

• int, whose values are 32-bit signed two's-complement integers

• long, whose values are 64-bit signed two's-complement integers

• char, whose values are 16-bit unsigned integers representing Unicode characters (§2.1)

• float, whose values are elements of the float value set or, where supported, the float-extended-exponent value set

• double, whose values are elements of the double value set or, where supported, the double-extended-exponent value set

The values of the returnAddress type are pointers to the opcodes of Java virtual machine instructions. Of the primitive types only the returnAddress type is not directly associated with a Java programming language type.

3.3.1 Integral Types and Values

• For byte, from -128 to 127 (-2⁷ to 2⁷-1), inclusive

• For short, from -32768 to 32767 (-2¹⁵ to 2¹⁵-1), inclusive

• For int, from -2147483648 to 2147483647 (-2³¹ to 2³¹-1), inclusive

• For long, from -9223372036854775808 to 9223372036854775807 (-2⁶³ to 2⁶³-1), inclusive

• For char, from 0 to 65535 inclusive

3.3.2 Floating-Point Types, Value Sets, and Values

The IEEE 754 standard includes not only positive and negative sign-magnitude numbers, but also positive and negative zeros, positive and negative infinities, and a special Not-a-Number value (hereafter abbreviated as "NaN"). The NaN value is used to represent the result of certain invalid operations such as dividing zero by zero.

Every implementation of the Java virtual machine is required to support two standard sets of floating-point values, called the float value set and the double value set. In addition, an implementation of the Java virtual machine may, at its option, support either or both of two extended-exponent floating-point value sets, called the float-extended-exponent value set and the double-extended-exponent value set. These extended-exponent value sets may, under certain circumstances, be used instead of the standard value sets to represent the values of type float or double.

The finite nonzero values of any floating-point value set can all be expressed in the form s · m· 2^{(e -N + 1)}, where s is +1 or -1, m is a positive integer less than 2^N, and e is an integer between Emin = - (2^{K -1}-2) and Emax = 2^{K -1}-1, inclusive, and where N and K are parameters that depend on the value set. Some values can be represented in this form in more than one way; for example, supposing that a value v in a value set might be represented in this form using certain values for s, m, and e, then if it happened that m were even and e were less than 2^{K -1}, one could halve m and increase e by 1 to produce a second representation for the same value v. A representation in this form is called normalized if m 2^{N -1}; otherwise the representation is said to be denormalized. If a value in a value set cannot be represented in such a way that m 2^{N -1}, then the value is said to be a denormalized value, because it has no normalized representation.

The constraints on the parameters N and K (and on the derived parameters Emin and Emax) for the two required and two optional floating-point value sets are summarized in Table 3.1.

Parameter	float	float-extended- exponent	double	double-extended- exponent
N	24	24	53	53
K	8	11	11	15
Emax	+127	+1023	+1023	+16383
Emin	-126	-1022	-1022	-16382

Where one or both extended-exponent value sets are supported by an implementation, then for each supported extended-exponent value set there is a specific implementation-dependent constant K, whose value is constrained by Table 3.1; this value K in turn dictates the values for Emin and Emax.

Each of the four value sets includes not only the finite nonzero values that are ascribed to it above, but also the five values positive zero, negative zero, positive infinity, negative infinity, and NaN.

Note that the constraints in Table 3.1 are designed so that every element of the float value set is necessarily also an element of the float-extended-exponent value set, the double value set, and the double-extended-exponent value set. Likewise, each element of the double value set is necessarily also an element of the double-extended-exponent value set. Each extended-exponent value set has a larger range of exponent values than the corresponding standard value set, but does not have more precision.

The elements of the float value set are exactly the values that can be represented using the single floating-point format defined in the IEEE 754 standard, except that there is only one NaN value (IEEE 754 specifies 2²⁴ - 2 distinct NaN values). The elements of the double value set are exactly the values that can be represented using the double floating-point format defined in the IEEE 754 standard, except that there is only one NaN value (IEEE 754 specifies 2⁵³ - 2 distinct NaN values). Note, however, that the elements of the float-extended-exponent and double-extended-exponent value sets defined here do not correspond to the values that be represented using IEEE 754 single extended and double extended formats, respectively. This specification does not mandate a specific representation for the values of the floating-point value sets except where floating-point values must be represented in the class file format (§4.4.4, §4.4.5).

The float, float-extended-exponent, double, and double-extended-exponent value sets are not types. It is always correct for an implementation of the Java virtual machine to use an element of the float value set to represent a value of type float; however, it may be permissible in certain contexts for an implementation to use an element of the float-extended-exponent value set instead. Similarly, it is always correct for an implementation to use an element of the double value set to represent a value of type double; however, it may be permissible in certain contexts for an implementation to use an element of the double-extended-exponent value set instead.

Except for NaNs, values of the floating-point value sets are ordered. When arranged from smallest to largest, they are negative infinity, negative finite values, positive and negative zero, positive finite values, and positive infinity.

Floating-point positive zero and floating-point negative zero compare as equal, but there are other operations that can distinguish them; for example, dividing 1.0 by 0.0 produces positive infinity, but dividing 1.0 by -0.0 produces negative infinity.

NaNs are unordered, so numerical comparisons and tests for numerical equality have the value false if either or both of their operands are NaN. In particular, a test for numerical equality of a value against itself has the value false if and only if the value is NaN. A test for numerical inequality has the value true if either operand is NaN.

3.3.3 The returnAddress Type and Values

3.3.4 The boolean Type

The Java virtual machine does directly support boolean arrays. Its newarray instruction enables creation of boolean arrays. Arrays of type boolean are accessed and modified using the byte array instructions baload and bastore.²

The Java virtual machine encodes boolean array components using 1 to represent true and 0 to represent false. Where Java programming language boolean values are mapped by compilers to values of Java virtual machine type int, the compilers must use the same encoding.

3.4 Reference Types and Values

The Java virtual machine specification does not mandate a concrete value encoding null.

3.5 Runtime Data Areas

3.5.1 The pc Register

3.5.2 Java Virtual Machine Stacks

The Java virtual machine specification permits Java virtual machine stacks either to be of a fixed size or to dynamically expand and contract as required by the computation. If the Java virtual machine stacks are of a fixed size, the size of each Java virtual machine stack may be chosen independently when that stack is created. A Java virtual machine implementation may provide the programmer or the user control over the initial size of Java virtual machine stacks, as well as, in the case of dynamically expanding or contracting Java virtual machine stacks, control over the maximum and minimum sizes.⁴

The following exceptional conditions are associated with Java virtual machine stacks:

• If the computation in a thread requires a larger Java virtual machine stack than is permitted, the Java virtual machine throws a StackOverflowError.

• If Java virtual machine stacks can be dynamically expanded, and expansion is attempted but insufficient memory can be made available to effect the expansion, or if insufficient memory can be made available to create the initial Java virtual machine stack for a new thread, the Java virtual machine throws an OutOfMemoryError.

3.5.3 Heap

The heap is created on virtual machine start-up. Heap storage for objects is reclaimed by an automatic storage management system (known as a garbage collector); objects are never explicitly deallocated. The Java virtual machine assumes no particular type of automatic storage management system, and the storage management technique may be chosen according to the implementor's system requirements. The heap may be of a fixed size or may be expanded as required by the computation and may be contracted if a larger heap becomes unnecessary. The memory for the heap does not need to be contiguous.

A Java virtual machine implementation may provide the programmer or the user control over the initial size of the heap, as well as, if the heap can be dynamically expanded or contracted, control over the maximum and minimum heap size.⁵

The following exceptional condition is associated with the heap:

• If a computation requires more heap than can be made available by the automatic storage management system, the Java virtual machine throws an OutOfMemoryError.

3.5.4 Method Area

The method area is created on virtual machine start-up. Although the method area is logically part of the heap, simple implementations may choose not to either garbage collect or compact it. This version of the Java virtual machine specification does not mandate the location of the method area or the policies used to manage compiled code. The method area may be of a fixed size or may be expanded as required by the computation and may be contracted if a larger method area becomes unnecessary. The memory for the method area does not need to be contiguous.

A Java virtual machine implementation may provide the programmer or the user control over the initial size of the method area, as well as, in the case of a varying-size method area, control over the maximum and minimum method area size.⁶

The following exceptional condition is associated with the method area:

• If memory in the method area cannot be made available to satisfy an allocation request, the Java virtual machine throws an OutOfMemoryError.

3.5.5 Runtime Constant Pool

Each runtime constant pool is allocated from the Java virtual machine's method area (§3.5.4). The runtime constant pool for a class or interface is constructed when the class or interface is created (§5.3) by the Java virtual machine.

The following exceptional condition is associated with the construction of the runtime constant pool for a class or interface:

• When creating a class or interface, if the construction of the runtime constant pool requires more memory than can be made available in the method area of the Java virtual machine, the Java virtual machine throws an OutOfMemoryError.

3.5.6 Native Method Stacks

The Java virtual machine specification permits native method stacks either to be of a fixed size or to dynamically expand and contract as required by the computation. If the native method stacks are of a fixed size, the size of each native method stack may be chosen independently when that stack is created. In any case, a Java virtual machine implementation may provide the programmer or the user control over the initial size of the native method stacks. In the case of varying-size native method stacks, it may also make available control over the maximum and minimum method stack sizes.⁷

The following exceptional conditions are associated with native method stacks:

• If the computation in a thread requires a larger native method stack than is permitted, the Java virtual machine throws a StackOverflowError.

• If native method stacks can be dynamically expanded and native method stack expansion is attempted but insufficient memory can be made available, or if insufficient memory can be made available to create the initial native method stack for a new thread, the Java virtual machine throws an OutOfMemoryError.

3.6 Frames

A new frame is created each time a method is invoked. A frame is destroyed when its method invocation completes, whether that completion is normal or abrupt (it throws an uncaught exception). Frames are allocated from the Java virtual machine stack (§3.5.2) of the thread creating the frame. Each frame has its own array of local variables (§3.6.1), its own operand stack (§3.6.2), and a reference to the runtime constant pool (§3.5.5) of the class of the current method.

The sizes of the local variable array and the operand stack are determined at compile time and are supplied along with the code for the method associated with the frame (§4.7.3). Thus the size of the frame data structure depends only on the implementation of the Java virtual machine, and the memory for these structures can be allocated simultaneously on method invocation.

Only one frame, the frame for the executing method, is active at any point in a given thread of control. This frame is referred to as the current frame, and its method is known as the current method. The class in which the current method is defined is the current class. Operations on local variables and the operand stack are typically with reference to the current frame.

A frame ceases to be current if its method invokes another method or if its method completes. When a method is invoked, a new frame is created and becomes current when control transfers to the new method. On method return, the current frame passes back the result of its method invocation, if any, to the previous frame. The current frame is then discarded as the previous frame becomes the current one.

Note that a frame created by a thread is local to that thread and cannot be referenced by any other thread.

3.6.1 Local Variables

A single local variable can hold a value of type boolean, byte, char, short, int, float, reference, or returnAddress. A pair of local variables can hold a value of type long or double.

Local variables are addressed by indexing. The index of the first local variable is zero. An integer is be considered to be an index into the local variable array if and only if that integer is between zero and one less than the size of the local variable array.

A value of type long or type double occupies two consecutive local variables. Such a value may only be addressed using the lesser index. For example, a value of type double stored in the local variable array at index n actually occupies the local variables with indices n and n +1; however, the local variable at index n +1 cannot be loaded from. It can be stored into. However, doing so invalidates the contents of local variable n.

The Java virtual machine does not require n to be even. In intuitive terms, values of types double and long need not be 64-bit aligned in the local variables array. Implementors are free to decide the appropriate way to represent such values using the two local variables reserved for the value.

The Java virtual machine uses local variables to pass parameters on method invocation. On class method invocation any parameters are passed in consecutive local variables starting from local variable 0. On instance method invocation, local variable 0 is always used to pass a reference to the object on which the instance method is being invoked (this in the Java programming language). Any parameters are subsequently passed in consecutive local variables starting from local variable 1.

3.6.2 Operand Stacks

Where it is clear by context, we will sometimes refer to the operand stack of the current frame as simply the operand stack.

The operand stack is empty when the frame that contains it is created. The Java virtual machine supplies instructions to load constants or values from local variables or fields onto the operand stack. Other Java virtual machine instructions take operands from the operand stack, operate on them, and push the result back onto the operand stack. The operand stack is also used to prepare parameters to be passed to methods and to receive method results.

For example, the iadd instruction adds two int values together. It requires that the int values to be added be the top two values of the operand stack, pushed there by previous instructions. Both of the int values are popped from the operand stack. They are added, and their sum is pushed back onto the operand stack. Subcomputations may be nested on the operand stack, resulting in values that can be used by the encompassing computation.

Each entry on the operand stack can hold a value of any Java virtual machine type, including a value of type long or type double.

Values from the operand stack must be operated upon in ways appropriate to their types. It is not possible, for example, to push two int values and subsequently treat them as a long or to push two float values and subsequently add them with an iadd instruction. A small number of Java virtual machine instructions (the dup instructions and swap) operate on runtime data areas as raw values without regard to their specific types; these instructions are defined in such a way that they cannot be used to modify or break up individual values. These restrictions on operand stack manipulation are enforced through class file verification (§4.9).

At any point in time an operand stack has an associated depth, where a value of type long or double contributes two units to the depth and a value of any other type contributes one unit.

3.6.3 Dynamic Linking

This late binding of the methods and variables makes changes in other classes that a method uses less likely to break this code.

3.6.4 Normal Method Invocation Completion

The current frame (§3.6) is used in this case to restore the state of the invoker, including its local variables and operand stack, with the program counter of the invoker appropriately incremented to skip past the method invocation instruction. Execution then continues normally in the invoking method's frame with the returned value (if any) pushed onto the operand stack of that frame.

3.6.5 Abrupt Method Invocation Completion

3.6.6 Additional Information

3.7 Representation of Objects

3.8 Floating-Point Arithmetic

3.8.1 Java Virtual Machine Floating-Point Arithmetic and IEEE 754

• The floating-point operations of the Java virtual machine do not throw exceptions, trap, or otherwise signal the IEEE 754 exceptional conditions of invalid operation, division by zero, overflow, underflow, or inexact. The Java virtual machine has no signaling NaN value.

• The Java virtual machine does not support IEEE 754 signaling floating-point comparisons.

• The rounding operations of the Java virtual machine always use IEEE 754 round to nearest mode. Inexact results are rounded to the nearest representable value, with ties going to the value with a zero least-significant bit. This is the IEEE 754 default mode. But Java virtual machine instructions that convert values of floating-point types to values of integral types round toward zero. The Java virtual machine does not give any means to change the floating-point rounding mode.

• The Java virtual machine does not support either the IEEE 754 single extended or double extended format, except insofar as the double and double-extended-exponent value sets may be said to support the single extended format. The float-extended-exponent and double-extended-exponent value sets, which may optionally be supported, do not correspond to the values of the IEEE 754 extended formats: the IEEE 754 extended formats require extended precision as well as extended exponent range.

3.8.2 Floating-Point Modes

Note that this mapping of the ACC_STRICT bit implies that methods in classes compiled by a compiler that predates the Java 2 platform, v1.2, are effectively not FP-strict.

We will refer to an operand stack as having a given floating-point mode when the method whose invocation created the frame containing the operand stack has that floating-point mode. Similarly, we will refer to a Java virtual machine instruction as having a given floating-point mode when the method containing that instruction has that floating-point mode.

If a float-extended-exponent value set is supported (§3.3.2), values of type float on an operand stack that is not FP-strict may range over that value set except where prohibited by value set conversion (§3.8.3). If a double-extended-exponent value set is supported (§3.3.2), values of type double on an operand stack that is not FP-strict may range over that value set except where prohibited by value set conversion.

In all other contexts, whether on the operand stack or elsewhere, and regardless of floating-point mode, floating-point values of type float and double may only range over the float value set and double value set, respectively. In particular, class and instance fields, array elements, local variables, and method parameters may only contain values drawn from the standard value sets.

3.8.3 Value Set Conversion

Where value set conversion is indicated, an implementation is permitted to perform one of the following operations on a value:

• If the value is of type float and is not an element of the float value set, it maps the value to the nearest element of the float value set.

• If the value is of type double and is not an element of the double value set, it maps the value to the nearest element of the double value set.

  In addition, where value set conversion is indicated certain operations are required:

• Suppose execution of a Java virtual machine instruction that is not FP-strict causes a value of type float to be pushed onto an operand stack that is FP-strict, passed as a parameter, or stored into a local variable, a field, or an element of an array. If the value is not an element of the float value set, it maps the value to the nearest element of the float value set.

• Suppose execution of a Java virtual machine instruction that is not FP-strict causes a value of type double to be pushed onto an operand stack that is FP-strict, passed as a parameter, or stored into a local variable, a field, or an element of an array. If the value is not an element of the double value set, it maps the value to the nearest element of the double value set.

Not all values from an extended-exponent value set can be mapped exactly to a value in the corresponding standard value set. If a value being mapped is too large to be represented exactly (its exponent is greater than that permitted by the standard value set), it is converted to a (positive or negative) infinity of the corresponding type. If a value being mapped is too small to be represented exactly (its exponent is smaller than that permitted by the standard value set), it is rounded to the nearest of a representable denormalized value or zero of the same sign.

Value set conversion preserves infinities and NaNs and cannot change the sign of the value being converted. Value set conversion has no effect on a value that is not of a floating-point type.

3.9 Specially Named Initialization Methods

A class or interface has at most one class or interface initialization method and is initialized (§2.17.4) by invoking that method. The initialization method of a class or interface is static and takes no arguments. It has the special name <clinit>. This name is supplied by a compiler. Because the name <clinit> is not a valid identifier, it cannot be used directly in a program written in the Java programming language. Class and interface initialization methods are invoked implicitly by the Java virtual machine; they are never invoked directly from any Java virtual machine instruction, but are invoked only indirectly as part of the class initialization process.

3.10 Exceptions

In cases where a finally clause (§2.16.2) is used, the finally clause is executed during the propagation of an exception thrown from the associated try block and any associated catch block, even if no catch clause that handles the thrown exception may be found.

As implemented by the Java virtual machine, each catch or finally clause of a method is represented by an exception handler. An exception handler specifies the range of offsets into the Java virtual machine code implementing the method for which the exception handler is active, describes the type of exception that the exception handler is able to handle, and specifies the location of the code that is to handle that exception. An exception matches an exception handler if the offset of the instruction that caused the exception is in the range of offsets of the exception handler and the exception type is the same class as or a subclass of the class of exception that the exception handler handles. When an exception is thrown, the Java virtual machine searches for a matching exception handler in the current method. If a matching exception handler is found, the system branches to the exception handling code specified by the matched handler.

If no such exception handler is found in the current method, the current method invocation completes abruptly (§3.6.5). On abrupt completion, the operand stack and local variables of the current method invocation are discarded, and its frame is popped, reinstating the frame of the invoking method. The exception is then rethrown in the context of the invoker's frame and so on, continuing up the method invocation chain. If no suitable exception handler is found before the top of the method invocation chain is reached, the execution of the thread in which the exception was thrown is terminated.

The order in which the exception handlers of a method are searched for a match is important. Within a class file the exception handlers for each method are stored in a table (§4.7.3). At run time, when an exception is thrown, the Java virtual machine searches the exception handlers of the current method in the order that they appear in the corresponding exception handler table in the class file, starting from the beginning of that table. Because try statements are structured, a compiler for the Java programming language can always order the entries of the exception handler table such that, for any thrown exception and any program counter value in that method, the first exception handler that matches the thrown exception corresponds to the innermost matching catch or finally clause.

Note that the Java virtual machine does not enforce nesting of or any ordering of the exception table entries of a method (§4.9.5). The exception handling semantics of the Java programming language are implemented only through cooperation with the compiler. When class files are generated by some other means, the defined search procedure ensures that all Java virtual machines will behave consistently.

More information on the implementation of catch and finally clauses is given in Chapter 7, "Compiling for the Java Virtual Machine."

3.11 Instruction Set Summary

Ignoring exceptions, the inner loop of a Java virtual machine interpreter is effectively

do {
     fetch an opcode;
     if (operands) fetch operands;
     execute the action for the opcode;
} while (there is more to do);

(byte1 << 8) | byte2

The decision to limit the Java virtual machine opcode to a byte and to forgo data alignment within compiled code reflects a conscious bias in favor of compactness, possibly at the cost of some performance in naive implementations. A one-byte opcode also limits the size of the instruction set. Not assuming data alignment means that immediate data larger than a byte must be constructed from bytes at run time on many machines.

3.11.1 Types and the Java Virtual Machine

For the majority of typed instructions, the instruction type is represented explicitly in the opcode mnemonic by a letter: i for an int operation, l for long, s for short, b for byte, c for char, f for float, d for double, and a for reference. Some instructions for which the type is unambiguous do not have a type letter in their mnemonic. For instance, arraylength always operates on an object that is an array. Some instructions, such as goto, an unconditional control transfer, do not operate on typed operands.

Given the Java virtual machine's one-byte opcode size, encoding types into opcodes places pressure on the design of its instruction set. If each typed instruction supported all of the Java virtual machine's runtime data types, there would be more instructions than could be represented in a byte. Instead, the instruction set of the Java virtual machine provides a reduced level of type support for certain operations. In other words, the instruction set is intentionally not orthogonal. Separate instructions can be used to convert between unsupported and supported data types as necessary.

Table 3.2 summarizes the type support in the instruction set of the Java virtual machine. A specific instruction, with type information, is built by replacing the T in the instruction template in the opcode column by the letter in the type column. If the type column for some instruction template and type is blank, then no instruction exists supporting that type of operation. For instance, there is a load instruction for type int, iload, but there is no load instruction for type byte.

Note that most instructions in Table 3.2 do not have forms for the integral types byte, char, and short. None have forms for the boolean type. Compilers encode loads of literal values of types byte and short using Java virtual machine instructions that sign-extend those values to values of type int at compile time or run time. Loads of literal values of types boolean and char are encoded using instructions that zero-extend the literal to a value of type int at compile time or run time. Likewise, loads from arrays of values of type boolean, byte, short, and char are encoded using Java virtual machine instructions that sign-extend or zero-extend the values to values of type int. Thus, most operations on values of actual types boolean, byte, char, and short are correctly performed by instructions operating on values of computational type int.

opcode	byte	short	int	long	float	double	char	reference
Tipush	bipush	sipush
Tconst			iconst	lconst	fconst	dconst		aconst
Tload			iload	lload	fload	dload		aload
Tstore			istore	lstore	fstore	dstore		astore
Tinc			iinc
Taload	baload	saload	iaload	laload	faload	daload	caload	aaload
Tastore	bastore	sastore	iastore	lastore	fastore	dastore	castore	aastore
Tadd			iadd	ladd	fadd	dadd
Tsub			isub	lsub	fsub	dsub
Tmul			imul	lmul	fmul	dmul
Tdiv			idiv	ldiv	fdiv	ddiv
Trem			irem	lrem	frem	drem
Tneg			ineg	lneg	fneg	dneg
Tshl			ishl	lshl
Tshr			ishr	lshr
Tushr			iushr	lushr
Tand			iand	land
Tor			ior	lor
Txor			ixor	lxor
i2T	i2b	i2s		i2l	i2f	i2d
l2T			l2i		l2f	l2d
f2T			f2i	f2l		f2d
d2T			d2i	d2l	d2f
Tcmp				lcmp
Tcmpl					fcmpl	dcmpl
Tcmpg					fcmpg	dcmpg
if_TcmpOP			if_icmpOP					if_acmpOP
Treturn			ireturn	lreturn	freturn	dreturn		areturn

The mapping between Java virtual machine actual types and Java virtual machine computational types is summarized by Table 3.3.

Actual Type	Computational Type	Category
boolean	int	category 1
byte	int	category 1
char	int	category 1
short	int	category 1
int	int	category 1
float	float	category 1
reference	reference	category 1
returnAddress	returnAddress	category 1
long	long	category 2
double	double	category 2

Certain Java virtual machine instructions such as pop and swap operate on the operand stack without regard to type; however, such instructions are constrained to use only on values of certain categories of computational types, also given in Table 3.3.

The remainder of this chapter summarizes the Java virtual machine instruction set.

3.11.2 Load and Store Instructions

• Load a local variable onto the operand stack: iload, iload_<n>, lload, lload_<n>, fload, fload_<n>, dload, dload_<n>, aload, aload_<n>.

• Store a value from the operand stack into a local variable: istore, istore_<n>, lstore, lstore_<n>, fstore, fstore_<n>, dstore, dstore_<n>, astore, astore_<n>.

• Load a constant onto the operand stack: bipush, sipush, ldc, ldc_w, ldc2_w, aconst_null, iconst_m1, iconst_<i>, lconst_<l>, fconst_<f>, dconst_<d>.

• Gain access to more local variables using a wider index, or to a larger immediate operand: wide.

Instruction mnemonics shown above with trailing letters between angle brackets (for instance, iload_<n>) denote families of instructions (with members iload_0, iload_1, iload_2, and iload_3 in the case of iload_<n>). Such families of instructions are specializations of an additional generic instruction (iload) that takes one operand. For the specialized instructions, the operand is implicit and does not need to be stored or fetched. The semantics are otherwise the same (iload_0 means the same thing as iload with the operand 0). The letter between the angle brackets specifies the type of the implicit operand for that family of instructions: for <n>, a nonnegative integer; for <i>, an int; for <l>, a long; for <f>, a float; and for <d>, a double. Forms for type int are used in many cases to perform operations on values of type byte, char, and short (§3.11.1).

This notation for instruction families is used throughout The Java^TM Virtual Machine Specification.

3.11.3 Arithmetic Instructions

• Add: iadd, ladd, fadd, dadd.

• Subtract: isub, lsub, fsub, dsub.

• Multiply: imul, lmul, fmul, dmul.

• Divide: idiv, ldiv, fdiv, ddiv.

• Remainder: irem, lrem, frem, drem.

• Negate: ineg, lneg, fneg, dneg.

• Shift: ishl, ishr, iushr, lshl, lshr, lushr.

• Bitwise OR: ior, lor.

• Bitwise AND: iand, land.

• Bitwise exclusive OR: ixor, lxor.

• Local variable increment: iinc.

• Comparison: dcmpg, dcmpl, fcmpg, fcmpl, lcmp.

The Java virtual machine does not indicate overflow during operations on integer data types. The only integer operations that can throw an exception are the integer divide instructions (idiv and ldiv) and the integer remainder instructions (irem and lrem), which throw an ArithmeticException if the divisor is zero.

Java virtual machine operations on floating-point numbers behave as specified in IEEE 754. In particular, the Java virtual machine requires full support of IEEE 754 denormalized floating-point numbers and gradual underflow, which make it easier to prove desirable properties of particular numerical algorithms.

The Java virtual machine requires that floating-point arithmetic behave as if every floating-point operator rounded its floating-point result to the result precision. Inexact results must be rounded to the representable value nearest to the infinitely precise result; if the two nearest representable values are equally near, the one having a least significant bit of zero is chosen. This is the IEEE 754 standard's default rounding mode, known as round to nearest mode.

The Java virtual machine uses the IEEE 754 round towards zero mode when converting a floating-point value to an integer. This results in the number being truncated; any bits of the significand that represent the fractional part of the operand value are discarded. Round towards zero mode chooses as its result the type's value closest to, but no greater in magnitude than, the infinitely precise result.

The Java virtual machine's floating-point operators do not throw runtime exceptions (not to be confused with IEEE 754 floating-point exceptions). An operation that overflows produces a signed infinity, an operation that underflows produces a denormalized value or a signed zero, and an operation that has no mathematically definite result produces NaN. All numeric operations with NaN as an operand produce NaN as a result.

Comparisons on values of type long (lcmp) perform a signed comparison. Comparisons on values of floating-point types (dcmpg, dcmpl, fcmpg, fcmpl) are performed using IEEE 754 nonsignaling comparisons.

3.11.4 Type Conversion Instructions

The Java virtual machine directly supports the following widening numeric conversions:

• int to long, float, or double

• long to float or double

• float to double

Conversion of an int or a long value to float, or of a long value to double, may lose precision, that is, may lose some of the least significant bits of the value; the resulting floating-point value is a correctly rounded version of the integer value, using IEEE 754 round to nearest mode.

A widening numeric conversion of an int to a long simply sign-extends the two's-complement representation of the int value to fill the wider format. A widening numeric conversion of a char to an integral type zero-extends the representation of the char value to fill the wider format.

Despite the fact that loss of precision may occur, widening numeric conversions never cause the Java virtual machine to throw a runtime exception (not to be confused with an IEEE 754 floating-point exception).

Note that widening numeric conversions do not exist from integral types byte, char, and short to type int. As noted in §3.11.1, values of type byte, char, and short are internally widened to type int, making these conversions implicit.

The Java virtual machine also directly supports the following narrowing numeric conversions:

• int to byte, short, or char

• long to int

• float to int or long

• double to int, long, or float

A narrowing numeric conversion of an int or long to an integral type T simply discards all but the N lowest-order bits, where N is the number of bits used to represent type T. This may cause the resulting value not to have the same sign as the input value.

In a narrowing numeric conversion of a floating-point value to an integral type T, where T is either int or long, the floating-point value is converted as follows:

• If the floating-point value is NaN, the result of the conversion is an int or long 0.

• Otherwise, if the floating-point value is not an infinity, the floating-point value is rounded to an integer value V using IEEE 754 round towards zero mode. There are two cases:
  • If T is long and this integer value can be represented as a long, then the result is the long value V.

  • If T is of type int and this integer value can be represented as an int, then the result is the int value V.

• Otherwise:
  • Either the value must be too small (a negative value of large magnitude or negative infinity), and the result is the smallest representable value of type int or long.

  • Or the value must be too large (a positive value of large magnitude or positive infinity), and the result is the largest representable value of type int or long.

Despite the fact that overflow, underflow, or loss of precision may occur, narrowing conversions among numeric types never cause the Java virtual machine to throw a runtime exception (not to be confused with an IEEE 754 floating-point exception).

3.11.5 Object Creation and Manipulation

• Create a new class instance: new.

• Create a new array: newarray, anewarray, multianewarray.

• Access fields of classes (static fields, known as class variables) and fields of class instances (non-static fields, known as instance variables): getfield, putfield, getstatic, putstatic.

• Load an array component onto the operand stack: baload, caload, saload, iaload, laload, faload, daload, aaload.

• Store a value from the operand stack as an array component: bastore, castore, sastore, iastore, lastore, fastore, dastore, aastore.

• Get the length of array: arraylength.

• Check properties of class instances or arrays: instanceof, checkcast.

3.11.6 Operand Stack Management Instructions

3.11.7 Control Transfer Instructions

• Conditional branch: ifeq, iflt, ifle, ifne, ifgt, ifge, ifnull, ifnonnull, if_icmpeq, if_icmpne, if_icmplt, if_icmpgt, if_icmple, if_icmpge, if_acmpeq, if_acmpne.

• Compound conditional branch: tableswitch, lookupswitch.

• Unconditional branch: goto, goto_w, jsr, jsr_w, ret.

Conditional branches on comparisons between data of types boolean, byte, char, and short are performed using int comparison instructions (§3.11.1). A conditional branch on a comparison between data of types long, float, or double is initiated using an instruction that compares the data and produces an int result of the comparison (§3.11.3). A subsequent int comparison instruction tests this result and effects the conditional branch. Because of its emphasis on int comparisons, the Java virtual machine provides a rich complement of conditional branch instructions for type int.

All int conditional control transfer instructions perform signed comparisons.

3.11.8 Method Invocation and Return Instructions

• invokevirtual invokes an instance method of an object, dispatching on the (virtual) type of the object. This is the normal method dispatch in the Java programming language.

• invokeinterface invokes a method that is implemented by an interface, searching the methods implemented by the particular runtime object to find the appropriate method.

• invokespecial invokes an instance method requiring special handling, whether an instance initialization method (§3.9), a private method, or a superclass method.

• invokestatic invokes a class (static) method in a named class.

3.11.9 Throwing Exceptions

3.11.10 Implementing finally

3.11.11 Synchronization

Method-level synchronization is handled as part of method invocation and return (see Section 3.11.8, "Method Invocation and Return Instructions").

Synchronization of sequences of instructions is typically used to encode the synchronized blocks of the Java programming language. The Java virtual machine supplies the monitorenter and monitorexit instructions to support such constructs.

Proper implementation of synchronized blocks requires cooperation from a compiler targeting the Java virtual machine. The compiler must ensure that at any method invocation completion a monitorexit instruction will have been executed for each monitorenter instruction executed since the method invocation. This must be the case whether the method invocation completes normally (§3.6.4) or abruptly (§3.6.5).

The compiler enforces proper pairing of monitorenter and monitorexit instructions on abrupt method invocation completion by generating exception handlers (§3.10) that will match any exception and whose associated code executes the necessary monitorexit instructions (§7.14).

3.12 Class Libraries

Classes that might require special support from the Java virtual machine include those that support:

• Reflection, such as the classes in the package java.lang.reflect and the class Class.

• Loading and creation of a class or interface. The most obvious example is the class ClassLoader.

• Linking and initialization of a class or interface. The example classes cited above fall into this category as well.

• Security, such as the classes in the package java.security and other classes such as SecurityManager.

• Multithreading, such as the class Thread.

• Weak references, such as the classes in the package java.lang.ref.⁹

3.13 Public Design, Private Implementation

It is important to understand where the line between the public design and the private implementation lies. A Java virtual machine implementation must be able to read class files and must exactly implement the semantics of the Java virtual machine code therein. One way of doing this is to take this document as a specification and to implement that specification literally. But it is also perfectly feasible and desirable for the implementor to modify or optimize the implementation within the constraints of this specification. So long as the class file format can be read and the semantics of its code are maintained, the implementor may implement these semantics in any way. What is "under the hood" is the implementor's business, as long as the correct external interface is carefully maintained.¹⁰

The implementor can use this flexibility to tailor Java virtual machine implementations for high performance, low memory use, or portability. What makes sense in a given implementation depends on the goals of that implementation. The range of implementation options includes the following:

• Translating Java virtual machine code at load time or during execution into the instruction set of another virtual machine.

• Translating Java virtual machine code at load time or during execution into the native instruction set of the host CPU (sometimes referred to as just-in-time, or JIT, code generation).

² In Sun's JDK releases 1.0 and 1.1, and the Java 2 SDK, Standard Edition, v1.2, boolean arrays in the Java programming language are encoded as Java virtual machine byte arrays, using 8 bits per boolean element.

³ In the first edition of this specification, the Java virtual machine stack was known as the Java stack.

⁴ In Sun's implementations of the Java virtual machine in JDK releases 1.0.2 and 1.1, and the Java 2 SDK, Standard Edition, v1.2, Java virtual machine stacks are discontiguous and are independently expanded as required by the computation. Those implementations do not free memory allocated for a Java virtual machine stack until the associated thread terminates. Expansion is subject to a size limit for any one stack. The Java virtual machine stack size limit may be set on virtual machine start-up using the "-oss" flag. The Java virtual machine stack size limit can be used to limit memory consumption or to catch runaway recursions.

⁵ Sun's implementations of the Java virtual machine in JDK releases 1.0.2 and 1.1, and the Java 2 SDK, Standard Edition, v1.2, dynamically expand the heap as required by the computation, but never contract the heap. The initial and maximum sizes may be specified on virtual machine start-up using the "-ms" and "-mx" flags, respectively.

⁶ Sun's implementation of the Java virtual machine in JDK release 1.0.2 dynamically expands the method area as required by the computation, but never contracts the method area. The Java virtual machine implementations in Sun's JDK release 1.1 and the Java 2 SDK, Standard Edition, v1.2 garbage collect the method area. In neither case is user control over the initial, minimum, or maximum size of the method area provided.

⁷ Sun's implementations of the Java virtual machine in JDK releases 1.0.2 and 1.1, and the Java 2 SDK, Standard Edition, v1.2, allocate fixed-size native method stacks of a single size. The size of the native method stacks may be set on virtual machine start-up using the "-ss" flag. The native method stack size limit can be used to limit memory consumption or to catch runaway recursions in native methods. Sun's implementations do not check for native method stack overflow.

⁸ In some of Sun's implementations of the Java virtual machine, a reference to a class instance is a pointer to a handle that is itself a pair of pointers: one to a table containing the methods of the object and a pointer to the Class object that represents the type of the object, and the other to the memory allocated from the heap for the object data.

⁹ Weak references were introduced in the Java 2 platform, v1.2.

¹⁰ There are some exceptions: debuggers, profilers, and just-in-time code generators can each require access to elements of the Java virtual machine that are normally considered to be "under the hood." Where appropriate, Sun is working with other Java virtual machine implementors and tools vendors to develop common interfaces to the Java virtual machine for use by such tools, and to promote those interfaces across the industry. Information on publicly available low-level interfaces to the Java virtual machine will be made available at http://java.sun.com.

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The Java^TM Virtual Machine Specification
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