The Java virtual machine does not mandate any particular internal structure for objects.8
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3.8 Floating-Point Arithmetic
The Java virtual machine incorporates a subset of the floating-point arithmetic specified in IEEE Standard for Binary Floating-Point Arithmetic (ANSI/IEEE Std. 754-1985, New York).
3.8.1 Java Virtual Machine Floating-Point Arithmetic and IEEE 754
The key differences between the floating-point arithmetic supported by the Java virtual machine and the IEEE 754 standard are:
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
Every method has a floating-point mode, which is either FP-strict or not FP-strict. The floating-point mode of a method is determined by the setting of the ACC_STRICT bit of the access_flags item of the method_info structure (§4.6) defining the method. A method for which this bit is set is FP-strict; otherwise, the method is not FP-strict.
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
An implementation of the Java virtual machine that supports an extended floating-point value set is permitted or required, under specified circumstances, to map a value of the associated floating-point type between the extended and the standard value sets. Such a value set conversion is not a type conversion, but a mapping between the value sets associated with the same type.
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.
Such required value set conversions may occur as a result of passing a parameter of a floating-point type during method invocation, including native method invocation; returning a value of a floating-point type from a method that is not FP-strict to a method that is FP-strict; or storing a value of a floating-point type into a local variable, a field, or an array in a method that is not FP-strict.
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.
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3.9 Specially Named Initialization Methods
At the level of the Java virtual machine, every constructor (§2.12) appears as an instance initialization method that has the special name
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
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3.10 Exceptions
In the Java programming language, throwing an exception results in an immediate nonlocal transfer of control from the point where the exception was thrown. This transfer of control may abruptly complete, one by one, multiple statements, constructor invocations, static and field initializer evaluations, and method invocations. The process continues until a catch clause (§2.16.2) is found that handles the thrown value. If no such clause can be found, the current thread exits.
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."
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3.11 Instruction Set Summary
A Java virtual machine instruction consists of a one-byte opcode specifying the operation to be performed, followed by zero or more operands supplying arguments or data that are used by the operation. Many instructions have no operands and consist only of an opcode.
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);
The number and size of the operands are determined by the opcode. If an 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
(byte1 << 8) | byte2
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 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
Most of the instructions in the Java virtual machine instruction set encode type information about the operations they perform. For instance, the iload instruction loads the contents of a local variable, which must be an int, onto the operand stack. The fload instruction does the same with a float value. 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 3.2 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
The load and store instructions transfer values between the local variables (§3.6.1) and the operand stack (§3.6.2) of a Java virtual machine frame (§3.6):
Load a local variable onto the operand stack: iload, iload_
Store a value from the operand stack into a local variable: istore, istore_
Load a constant onto the operand stack: bipush, sipush, ldc, ldc_w, ldc2_w, aconst_null, iconst_m1, iconst_, lconst_
Gain access to more local variables using a wider index, or to a larger immediate operand: wide.
Instructions that access fields of objects and elements of arrays (§3.11.5) also transfer data to and from the operand stack.
Instruction mnemonics shown above with trailing letters between angle brackets (for instance, iload_
This notation for instruction families is used throughout The JavaTM Virtual Machine Specification.
3.11.3 Arithmetic Instructions
The arithmetic instructions compute a result that is typically a function of two values on the operand stack, pushing the result back on the operand stack. There are two main kinds of arithmetic instructions: those operating on integer values and those operating on floating-point values. Within each of these kinds, the arithmetic instructions are specialized to Java virtual machine numeric types. There is no direct support for integer arithmetic on values of the byte, short, and char types (§3.11.1), or for values of the boolean type; those operations are handled by instructions operating on type int. Integer and floating-point instructions also differ in their behavior on overflow and divide-by-zero. The arithmetic instructions are as follows:
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 semantics of the Java programming language operators on integer and floating-point values (§2.4.2, §2.4.4) are directly supported by the semantics of the Java virtual machine instruction set.
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 type conversion instructions allow conversion between Java virtual machine numeric types. These may be used to implement explicit conversions in user code or to mitigate the lack of orthogonality in the instruction set of the Java virtual machine.
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
The widening numeric conversion instructions are i2l, i2f, i2d, l2f, l2d, and f2d. The mnemonics for these opcodes are straightforward given the naming conventions for typed instructions and the punning use of 2 to mean "to." For instance, the i2d instruction converts an int value to a double. Widening numeric conversions do not lose information about the overall magnitude of a numeric value. Indeed, conversions widening from int to long and int to double do not lose any information at all; the numeric value is preserved exactly. Conversions widening from float to double that are FP-strict (§3.8.2) also preserve the numeric value exactly; however, such conversions that are not FP-strict may lose information about the overall magnitude of the converted value.
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
The narrowing numeric conversion instructions are i2b, i2c, i2s, l2i, f2i, f2l, d2i, d2l, and d2f. A narrowing numeric conversion can result in a value of different sign, a different order of magnitude, or both; it may thereby lose precision.
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.
A narrowing numeric conversion from double to float behaves 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 float is converted to a positive or negative zero of type float; a value too large to be represented as a float is converted to a positive or negative infinity. A double NaN is always converted to a float NaN.
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
Although both class instances and arrays are objects, the Java virtual machine creates and manipulates class instances and arrays using distinct sets of instructions:
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
A number of instructions are provided for the direct manipulation of the operand stack: pop, pop2, dup, dup2, dup_x1, dup2_x1, dup_x2, dup2_x2, swap.
3.11.7 Control Transfer Instructions
The control transfer instructions conditionally or unconditionally cause the Java virtual machine to continue execution with an instruction other than the one following the control transfer instruction. They are:
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.
The Java virtual machine has distinct sets of instructions that conditionally branch on comparison with data of int and reference types. It also has distinct conditional branch instructions that test for the null reference and thus is not required to specify a concrete value for null (§3.4).
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
The following four instructions invoke methods:
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.
The method return instructions, which are distinguished by return type, are ireturn (used to return values of type boolean, byte, char, short, or int), lreturn, freturn , dreturn, and areturn. In addition, the return instruction is used to return from methods declared to be void, instance initialization methods, and class or interface initialization methods.
3.11.9 Throwing Exceptions
An exception is thrown programmatically using the athrow instruction. Exceptions can also be thrown by various Java virtual machine instructions if they detect an abnormal condition.
3.11.10 Implementing finally
The implementation of the finally keyword uses the jsr, jsr_w, and ret instructions. See Section 4.9.6, "Exceptions and finally," and Section 7.13, "Compiling finally."
3.11.11 Synchronization
The Java virtual machine supports synchronization of both methods and sequences of instructions within a method using a single synchronization construct: the monitor .
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).
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3.12 Class Libraries
The Java virtual machine must provide sufficient support for the implementation of the class libraries of the associated platform. Some of the classes in these libraries cannot be implemented without the cooperation of the Java virtual machine.
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
The list above is meant to be illustrative rather than comprehensive. An exhaustive list of these classes or of the functionality they provide is beyond the scope of this book. See the specifications of the Java and Java 2 platform class libraries for details.
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3.13 Public Design, Private Implementation
Thus far this book has sketched the public view of the Java virtual machine: the class file format and the instruction set. These components are vital to the hardware-, operating system-, 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 hosts each implementing the Java or Java 2 platform, 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. 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).
The existence of a precisely defined virtual machine and object file format need not significantly restrict the creativity of the implementor. The Java virtual machine is designed to support many different implementations, providing new and interesting solutions while retaining compatibility between implementations.
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1 The first edition of The JavaTM Virtual Machine Specification did not consider boolean to be a Java virtual machine type. However, boolean values do have limited support in the Java virtual machine. This second edition clarifies the issue by treating boolean as a type.
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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