|Contents | Prev | Next | Index||The JavaTM Virtual Machine Specification|
This book specifies an abstract machine. It does not document any particular implementation of the Java Virtual Machine, including Sun's.
To implement the Java Virtual Machine correctly, you need only be able to read the Java
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, and will only be provided to make the exposition clearer. For example, the memory layout of runtime data areas, the garbage-collection algorithm used, and any optimizations of the bytecodes (for example, translating them into machine code) are left to the discretion of the implementor.
The Java Virtual Machine expects that nearly all type checking is done at compile time, not by the Java Virtual Machine itself. In particular, data need not be tagged or otherwise be inspectable to determine 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,
dadd are all Java Virtual Machine instructions that add two numeric values, but they require operands whose types are
double, respectively. For a summary of type support in the Java Virtual Machine's 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 may exist to an object. Although the Java Virtual Machine performs operations on objects, it never addresses them directly. Objects are always operated on, passed, and tested via values of type
returnAddresstype. The numeric types consist of the integral types:
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 version 1.1.5 characters (§2.1)
float, whose values are 32-bit IEEE 754 floating-point numbers
double, whose values are 64-bit IEEE 754 floating-point numbers
returnAddresstype are pointers to the opcodes of Java Virtual Machine instructions. Only the
returnAddresstype is not a Java language type.
byte, from -
127(-27 to 27-1), inclusive
short, from -
32767(-215 to 215-1), inclusive
int, from -
2147483647(-231 to 231-1), inclusive
long, from -
9223372036854775807(-263 to 263-1), inclusive
charis unsigned, so
65535when used in expressions, not -
doublerepresent single-precision 32-bit and double-precision 64- bit format IEEE 754 values as specified in IEEE Standard for Binary Floating-Point Arithmetic, ANSI/IEEE Std. 754-1985 (IEEE, New York).
The IEEE 754 standard includes not only positive and negative sign-magnitude numbers, but also positive and negative zeroes, positive and negative infinities, and a special Not-a-Number (hereafter abbreviated NaN) value that is used to represent the result of certain operations such as dividing zero by zero. Such values exist for both
The finite nonzero values of type
float are of the form s xfa m xfa 2e, where s is +1 or -1, m is a positive integer less than 224, and e is an integer between -149 and 104, inclusive. The largest positive finite floating-point literal of type
3.40282347e+38F. The smallest positive nonzero floating-point literal of type
The finite nonzero values of type
double are of the form s xfa m xfa 2e, where s is +1 or -1, m is a positive integer less than 253, and e is an integer between -1075 and 970, inclusive. The largest positive finite floating-point literal of type
1.79769313486231570e+308. The smallest positive nonzero floating-point literal of type
Floating-point positive zero and floating-point negative zero compare as equal, but there are other operations that can distinguish them; for example, dividing
0.0 produces positive infinity, but dividing
-0.0 produces negative infinity.
Except for NaN, floating-point values are ordered. When arranged from smallest to largest, they are negative infinity, negative finite values, negative zero, positive zero, positive finite values, and positive infinity.
NaN is unordered, so numerical comparisons have the value false if either or both of their operands are NaN. A test for numerical equality has the value false if either operand is NaN, and a test for numerical inequality has the value true if either operand is 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.
IEEE 754 defines a large number of distinct NaN values but fails to specify which NaN values are produced in various situations. To avoid portability problems, the Java Virtual Machine coalesces these NaN values together into a single conceptual NaN value.
returnAddressType and Values
returnAddresstype is used by the Java Virtual Machine's jsr, ret, and jsr_w instructions. The values of the
returnAddresstype are pointers to the opcodes of Java Virtual Machine instructions. Unlike the numeric primitive types, the
returnAddresstype does not correspond to any Java data type.
booleantype, the Java Virtual Machine does not have instructions dedicated to operations on
booleanvalues. Instead, a Java expression that operates on
booleanvalues is compiled to use the
intdata type to represent
Although the Java Virtual Machine has support for the creation of arrays of type
boolean (see the description of the newarray instruction), it does not have dedicated support for accessing and modifying elements of
boolean arrays. Arrays of type
boolean are accessed and modified using the
byte array instructions.1
For more information on the treatment of
boolean values in the Java Virtual Machine, see Chapter 7, "Compiling for the Java Virtual Machine."
referencetypes: class types, interface types, and array types, whose values are references to dynamically created class instances, arrays, or class instances or arrays that implement interfaces. A
referencevalue may also be the special null reference, a reference to no object, which will be denoted here by
nullreference initially has no runtime type, but may be cast to any type (§2.4).
returnAddress, or to hold a native pointer. Two words are large enough to hold values of the larger types,
double. Java's runtime data areas are all defined in terms of these abstract words.
A word is usually the size of a pointer on the host platform. On a 32-bit platform, a word is 32 bits, pointers are 32 bits, and
doubles naturally take up two words. A naive 64-bit implementation of the Java Virtual Machine may waste half of a word used to store a 32-bit datum, but may also be able to store all of a
long or a
double in one of the two words allotted to it.
The choice of a specific word size, although platform-specific, is made at the implementation level, not as part of the Java Virtual Machine's design. It is not visible outside the implementation or to code compiled for the Java Virtual Machine.
Throughout this book, all references to a word datum are to this abstract notion of a word.
pc(program counter) register. At any point, each Java Virtual Machine thread is executing the code of a single method, the current method (§3.6) for that thread. If that method is not
pcregister contains the address of the Java Virtual Machine instruction currently being executed. If the method currently being executed by the thread is
native, the value of the Java Virtual Machine's
pcregister is undefined. The Java Virtual Machine's
pcregister is one word wide, the width guaranteed to hold a
returnAddressor a native pointer on the specific platform.
The Java Virtual Machine specification permits Java stacks to be of either a fixed or a dynamically varying size. If the Java stacks are of a fixed size, the size of each Java 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 stacks, as well as, in the case of dynamically expanding or contracting Java stacks, control over the maximum and minimum Java stack sizes.
The following exceptional conditions are associated with Java stacks:
-oss" flag. The Java stack size limit can be used to limit memory consumption or to catch runaway recursions.
The Java heap is created on virtual machine start-up. Heap storage for objects is reclaimed by an automatic storage management system (typically 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 Java 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 Java 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.
The following exceptional condition is associated with the Java heap:
-ms" and "
-mx" flags, respectively.
The method area is created on virtual machine start-up. Although the method area is logically part of the garbage-collected heap, simple implementations may choose to neither garbage collect nor 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.
The following exceptional condition is associated with the method area:
constant_pooltable in a Java
classfile (§4.4). It contains several kinds of constants, ranging from numeric literals known at compile time to method and field references that must be resolved at run time. The constant pool serves a function similar to that of a symbol table for a conventional programming language, although it contains a wider range of data than a typical symbol table.
Each constant pool is allocated from the Java Virtual Machine's method area (§3.5.4). The constant pool for a class or interface is created when a Java
class file for the class or interface is successfully loaded (§2.16.2) by a Java Virtual Machine.
The following exceptional condition is associated with the creation of the constant pool for a class or interface:
classfile, if the creation of the 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
nativemethods, methods written in languages other than Java. A native method stack may also be used to implement an emulator for the Java Virtual Machine's instruction set in a language such as C. Implementations that do not support
nativemethods, and that do not themselves rely on conventional stacks, need not supply native method stacks. If supplied, native method stacks are typically allocated on a per thread basis when each thread is created.
The Java Virtual Machine specification permits native method stacks to be of either a fixed or a dynamically varying size. 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.
The following exceptional conditions are associated with Java stacks:
-ss" flag. The native method stack size limit can be used to limit memory consumption or to catch runaway recursions in
Sun's implementation does not currently check for native method stack overflow.
A new frame is created each time a Java method is invoked. A frame is destroyed when its method completes, whether that completion is normal or abnormal (by throwing an exception). Frames are allocated from the Java stack (§3.5.2) of the thread creating the frame. Each frame has its own set of local variables (§3.6.1) and its own operand stack (§3.6.2). The memory space for these structures can be allocated simultaneously, since the sizes of the local variable area and operand stack are known at compile time and the size of the frame data structure depends only upon the implementation of the Java Virtual Machine.
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 always are 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. Java Virtual Machine frames may be naturally thought of as being allocated on a stack, with one stack per Java thread (§2.17), but they may also be heap allocated.
Note that a frame created by a thread is local to that thread and cannot be directly referenced by any other thread.
Local variables are always one word wide. Two local variables are reserved for each
double value. These two local variables are addressed by the index of the first of the variables.
For example, a local variable with index n and containing a value of type
double actually occupies the two words at local variable indices n and n+1. The Java Virtual Machine does not require n to be even. (In intuitive implementation terms, 64-bit values need not be 64-bit aligned in the local variables array.) Implementors are free to decide the appropriate way to divide a 64-bit data value between two local variables.
For example, the iadd instruction adds two
int values together. It requires that the
int values to be added be the top two words 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 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 is one word wide. Values of types
double are pushed onto the operand stack as two words. The Java Virtual Machine does not require 64-bit values on the operand stack to be 64-bit aligned. Implementors are free to decide the appropriate way to divide a 64-bit data value between two operand stack words.
Values from the operand stack must be operated upon in ways appropriate to their types. It is incorrect, for example, to push two
int values and then treat them as a
long, or to push two
float values then add them with an iadd instruction. A small number of Java Virtual Machine instructions (the dup instructions and swap) operate on run-time data areas as raw values of a given width without regard to type; these instructions must not be used to break up or rearrange the words of 64-bit data. These restrictions on operand stack manipulation are enforced, in the Sun implementation, by the
class file verifier (§4.9).
classfile code for a method refers to methods to be invoked and variables to be accessed via symbolic references. Dynamic linking translates these symbolic method references into concrete method references, loading classes as necessary to resolve as-yet-undefined symbols, and translates variable accesses into appropriate offsets in storage structures associated with the runtime location of these variables.
This late binding of the methods and variables makes changes in other classes that a method uses less likely to break this code.
throwstatement. If the invocation of the current method completes normally, then a value may be returned to the invoking method. This occurs when the invoked method executes one of the return instructions (§3.11.8), the choice of which must be appropriate for the type of the value being returned (if any).
The Java Virtual Machine frame 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.
throwstatement also causes an exception to be thrown and, if the exception is not caught by the current method, results in abnormal method completion. A method invocation that completes abnormally never returns a value to its invoker.
Classobject that represents the type of the object, and the other to the memory allocated from the Java heap for the object data.
Other Java Virtual Machine implementations may use techniques such as inline caching rather than method table dispatch, and they may or may not use handles.
<init>. This name is supplied by a Java compiler. Because the name
<init>is not a valid identifier, it cannot be used directly by a Java programmer. Instance initialization methods may only be invoked within the Java Virtual Machine by the invokespecial instruction, and they may only be invoked on uninitialized class instances. An instance initialization method takes on the access permissions (§2.7.8) of the constructor from which it was derived.
At the level of the Java Virtual Machine, a class or interface is initialized (§2.16.4) by invoking its class or interface initialization method with no arguments. The initialization method of a class or interface has the special name
<clinit>. This name is supplied by a Java compiler. Because the name
<clinit> is not a valid identifier, it cannot be used directly by a Java programmer. Class and interface initialization methods are invoked implicitly by the Java Virtual Machine; they are never invoked directly from Java code or directly from any Java Virtual Machine instruction, but are only invoked indirectly as part of the class initialization process.
catchclause (§2.15.2) is found that catches the thrown value.
If no such
catch clause is found in the current method, then the current method invocation completes abnormally (§3.6.5). Its operand stack and local variables 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
catch clause is found before the top of the method invocation chain is reached, the execution of the thread that threw the exception is terminated.
At the level of the Java Virtual Machine, each
catch clause describes the Java Virtual Machine instruction range for which it is active, describes the types of exceptions that it is to handle, and gives the address of the code to handle it. An exception matches a
catch clause if the instruction that caused the exception is in the appropriate instruction range, and the exception type is the same type as or a subclass of the class of exception that the
catch clause handles. If a matching
catch clause is found, the system branches to the specified handler. If no handler is found, the process is repeated until all the nested
catch clauses of the current method have been exhausted.
The order of the
catch clauses in the list is important. The Java Virtual Machine execution continues at the first matching
catch clause. Because Java code is structured, it is always possible to arrange all the exception handlers for one method in a single list. For any possible program counter value, this list can be searched to find the proper exception handler, that is, the innermost exception handler that both contains the program counter value and can handle the exception being thrown.
If there is no matching
catch clause, the current method is said to have an uncaught exception. The execution state of the invoker, the method that invoked this method, is restored. The propagation of the exception continues as though the exception had occurred in the invoker at the instruction that invoked the method actually raising the exception.
Java supports more sophisticated forms of exception handling through its
try-catch-finally statements. In such forms, the
finally statement is executed even if no matching
catch clause is found. The way the Java Virtual Machine supports implementation of these forms is discussed in Chapter 7, "Compiling for the Java Virtual Machine."
classfile format. Given the aims of the Java Virtual Machine, the definition of this file format is of importance equal to its other components. The
classfile format precisely defines the contents of such a file, including details such as byte ordering that might be taken for granted in a platform-specific object file format.
Chapter 4, "The class File Format," covers the
class file format in detail.
Ignoring exceptions, the inner loop of the Java Virtual Machine execution is effectively
The number and size of the additional operands are determined by the opcode. If an additional operand is more than one byte in size, then it is stored in big-endian order-high-order byte first. For example, an unsigned 16-bit index into the local variables is stored as two unsigned bytes byte1 and byte2 such that its value is
fetch an opcode;
if (operands) fetch operands;
execute the action for the opcode;
} while (there is more to do);
The bytecode instruction stream is only single-byte aligned. The two exceptions are the tableswitch and lookupswitch instructions, which are padded to force internal alignment of some of their operands on 4-byte boundaries.
The decision to limit the Java Virtual Machine opcode to a byte and to forego 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 precludes certain implementation techniques that could improve the performance of a Java Virtual Machine emulator, and it 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.
int, onto the operand stack. The fload instruction does the same with a
floatvalue. The two instructions may have identical implementations, but have distinct opcodes.
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 3.1 summarizes the type support in the instruction set of the Java Virtual Machine. Only instructions that exist for multiple types are listed. 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
Note that most instructions in Table 3.1 do not have forms for the integral types
short. When writing to its local variables or operand stacks, the Java Virtual Machine internally sign-extends values of types
short to type
int, and zero-extends values of type
char to type
int. Thus, most operations on values of types
short are correctly performed by instructions operating on values of type
int. The Java Virtual Machine also treats values of Java type
boolean specially, as noted in §3.2.4.
The mapping between Java storage types and Java Virtual Machine computatational types is summarized by Table 3.2.
|Java (Storage) Type|
The exception to this mapping is in the case of arrays. Arrays of type
short can be directly represented by the Java Virtual Machine. Arrays of type
short are accessed using instructions specialized to those types. Arrays of type
boolean are accessed using
byte array instructions.
The remainder of this chapter summarizes the Java Virtual Machine instruction set.
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 natural number, 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
This notation for instruction families is used throughout The Java Virtual Machine Specification.
chartypes (§3.11.1); those operations are handled by instructions operating on type
int. Integer and floating-point instructions also differ in their behavior on overflow, underflow, and divide-by-zero. The arithmetic instructions are as follows:
The Java Virtual Machine does not indicate overflow or underflow 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 exactly 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 with its least significant bit zero is chosen. This is the IEEE 754 standard's default rounding mode, known as round-to-nearest.
The Java Virtual Machine uses round-towards-zero when converting a floatingpoint 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 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 produce no exceptions. An operation that overflows produces a signed infinity, an operation that underflows produces 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.
The Java Virtual Machine directly supports the following widening numeric conversions, a subset of Java's widening primitive conversions (§2.6.2):
intvalue to a
double. Widening numeric conversions do not lose information about the overall magnitude of a numeric value. Indeed, conversions widening from the
inttype to the
longtype and from
doubledo not lose any information at all; the numeric value is preserved exactly. Conversion of an
float, or of a
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.
According to this rule, 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 result in a runtime exception.
Note that widening numeric conversions do not exist from integral types
short to type
int. As noted in §3.11.1, values of type
short are internally widened to type
int, making these conversions implicit.
The Java Virtual Machine also directly supports the following narrowing numeric conversions, a subset of Java's narrowing primitive conversions (§2.6.3):
A narrowing numeric conversion of an
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
long, the floating-point value is converted to type T as follows:
longand this integer value can be represented as a
long, then the result is the
intand this integer value can be represented as an
int, then the result is the
floatbehaves in accordance with IEEE 754. The result is correctly rounded using IEEE 754 round-to-nearest mode. A value too small to be represented as a
floatis converted to a positive or negative zero of type
float; a value too large to be represented as a
floatis converted to a positive or negative infinity. A
doubleNaN is always converted to a
Despite the fact that overflow, underflow, or loss of precision may occur, narrowing conversions among numeric types never result in a runtime exception.
staticfields, known as class variables) and fields of class instances (non-
staticfields, known as instance variables): getfield, putfield, getstatic, putstatic.
referencetypes. Comparison with data of
shorttypes is done using an
intcomparison instruction (§3.11.1). Because of this added emphasis on
intcomparisons, the Java Virtual Machine includes a larger complement of conditional branch instructions for type
intthan for other types. The Java Virtual Machine has distinct conditional branch instructions that test for the null reference, and thus is not required to specify a concrete value for
long conditional control transfer instructions perform signed comparisons. Floating-point comparison is performed in accordance with IEEE 754.
privatemethod, or a superclass method: invokespecial.
static) method in a named class: invokestatic.
int), lreturn, freturn, dreturn, and areturn. In addition, the return instruction is used to return from methods declared to be
finallykeyword uses the jsr, jsr_w, and ret instructions. See Section 4.9.6, "Exceptions and finally" and Section 7.13, "Compiling finally."
classfile format and the instruction set. These components are vital to the platform- and implementation-independence of the Java Virtual Machine. The implementor may prefer to think of them as a means to securely communicate fragments of programs between two platforms, rather than as a blueprint to be followed exactly.
It is important to understand where the line between the public design and the private implementation lies. The Java Virtual Machine must be able to read
class files, and it 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.2
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:
classfile verifier; see Section 4.9, "Verification of class Files").
booleanarrays are effectively
bytearrays, using 8 bits per boolean element.
2 There are some exceptions: debuggers and JIT code generators can require access to elements of the Java Virtual Machine that are normally considered to be "under the hood." Sun is working with other Java Virtual Machine implementors and tools vendors to standardize interfaces to the Java Virtual Machine for use by such tools.
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