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Appendix A -- Inside the Java Virtual Machine

Appendix A

Inside the Java Virtual Machine


CONTENTS




Looking at a virtual machine from the outside in is probably the best way to understand its workings. Incremental learning results when you move from the known to the unknown, so this section starts with the item you are most familiar with: the class file.

The Class File

The class file is similar to standard language object modules. When a C language file is compiled, the output is an object module. Multiple object modules are linked together to form an executable program. In Java, the class file replaces the object module and the Java virtual machine replaces the executable program.

You'll find all the information needed to execute a class contained within the class file, as well as extra information that aids debugging and source file tracking. Remember that Java has no "header" include files, so the class file format also has to fully convey class layout and members. Parsing a class file yields a wealth of class information, not just its runtime architecture.

Layout

The overall layout uses an outer structure and a series of substructures that contain an ever-increasing amount of detail. The outer layer is described by the following structure:

ClassFile
{
    u4 magic;
    u2 minor_version;
    u2 major_version;
    u2 constant_pool_count;
    cp_info constant_pool[constant_pool_count - 1];
    u2 access_flags;
    u2 this_class;
    u2 super_class;
    u2 interfaces_count;
    u2 interfaces[interfaces_count];
    u2 fields_count;
    field_info fields[fields_count];
    u2 methods_count;
    method_info methods[methods_count];
    u2 attributes_count;
    attribute_info attributes[attribute_count];
}

In addition to the generic class information (this_class, super_class, version, and so forth), there are three major substructures: constant_pool, fields, and methods. Attributes are considered minor substructures because they recur throughout the class file at various levels. Fields and methods contain their own attributes. Some individual attributes also contain their own private attribute arrays.

The u2 and u4 types represent unsigned 2-byte and 4-byte quantities.

This simple class will be used throughout this appendix as the basis for class file exploration.

public class test
{
    public static int st_one;

    public test()
    {
        st_one = 100;
    }

    public test(int v)
    {
        st_one = v;
    }

    public native boolean getData(int data[] );

    public int do_countdown()
    {
        int x = st_one;

        System.out.println("Performing countdown:");
        while ( x- != 0 )
            System.out.println(x);
        return st_one;
    }

    public int do_countdown(int x)
    {
        int save = x;

        System.out.println("Performing countdown:");
        while ( x- != 0 )
            System.out.println(x);
        return save;
    }
}

This class doesn't actually do very much, but it does provide a basis for class file exploration. Once compiled, the outer layer of the resulting class file is as follows:

file name: test.class
      magic: cafebabe
    version: 45.3
  constants: Valid indexes: 1 - 39
     access: PUBLIC
this class: Class: test
super class: Class: java/lang/Object
interfaces: none
     fields: [1] st_one
    methods: [4] <init> <init> do_countdown do_countdown
 attributes: SourceFile(test.java)

Note
The above output was generated with a Java tool I wrote called ClassView. The full source code is available on this book's CD-ROM. ClassView parses and outputs the various features and levels of a given class file. It also provides a good basis for other class file parsers, such as disassemblers.

The magic number of a class is 0xcafebabe. This number must appear in a class file, or the file is assumed to be invalid. The current major version is 45; the current minor version is 3.

Access Flags

Access flags are used throughout the class file to convey the access characteristics of various items. The flag itself is a collection of 11 individual bits. Table A.1 lays out the masks.

Table A.1. Access flag bit values.

Flag ValueIndication
Acc_PUBLIC = 0x0001 Global visibility
Acc_PRIVATE = 0x0002 Local class visibility
Acc_PROTECTED = 0x0004 Subclass visibility
Acc_STATIC = 0x0008 One occurrence in system (not per class)
Acc_FINAL = 0x0010 No changes allowed
Acc_SYNchRONIZED = 0x0020 Access with a monitor
Acc_VOLATILE = 0x0040 No local caching
Acc_TRANSIENT = 0x0080 Not a persistent value
Acc_NATIVE = 0x0100 Native method implementation
Acc_INTERFACE = 0x0200 Class is an interface
Acc_ABSTRACT = 0x0400 Class or method is abstract

Access flags are present for a class and its fields and methods. Only a subset of values appears in any given item. Some bits apply only to fields (VOLATILE and TRANSIENT); others apply only to methods (SYNchRONIZED and NATIVE).

Attributes

Attributes, like access flags, appear throughout a class file. They have the following form:

GenericAttribute_info
{
    u2 attribute_name;
    u4 attribute_length;
    u1 info[attribute_length];
}

A generic structure exists to enable loaders to skip over attributes they don't understand. The actual attribute has a unique structure that can be read if the loader understands the format. As an example, the following structure specifies the format of a source file attribute:

SourceFile_attribute
{
    u2 attribute_name;
    u4 attribute_length;
    u2 sourcefile_index;
}

The name of an attribute is an index into the constant pool. You'll learn about constant pools momentarily. If a loader did not understand the source file attribute structure, it could skip the data by reading the number of bytes specified in the length parameter. For the source file attribute, the length is 2.

Constant Pool

The constant pool forms the basis for all numbers and strings within a class file. Nowhere else do you ever find strings or numbers in a class file. Any time there is a need to reference a string or number, an index into the constant pool is substituted. Consequently, the constant pool is the dominant feature of a class. The pool is even used directly within the virtual machine itself.

There are twelve different types of constants:

Each constant structure leads off with a tag identifying the structure type. Following the type is data specific to each individual structure. The layout of each constant structure follows:

CONSTANT_Utf8_info
{
    u1 tag;
    u2 length;
    u1 bytes[length];
}

CONSTANT_Unicode_info
{
    u1 tag;
    u2 length;
    u2 words[length];
}

CONSTANT_Integer_info
{
    u1 tag;
    u4 bytes;
}

CONSTANT_Float_info
{
    u1 tag;
    u4 bytes;
}

CONSTANT_Long_info
{
    u1 tag;
    u4 high_bytes;
    u4 low_bytes;
}

CONSTANT_Double_info
{
    u1 tag;
    u4 high_bytes;
    u4 low_bytes;
}

CONSTANT_Class_info
{
    u1 tag;
    u2 name_index;
}

CONSTANT_String_info
{
    u1 tag;
    u2 string_index;
}

CONSTANT_Fieldref_info
{
    u1 tag;
    u2 class_index;
    u2 name_and_type_index;
}

CONSTANT_Methodref_info
{
    u1 tag;
    u2 class_index;
    u2 name_and_type_index;
}

CONSTANT_InterfaceMethodref_info
{
    u1 tag;
    u2 class_index;
    u2 name_and_type_index;
}

CONSTANT_NameAndType_info
{
    u1 tag;
    u2 name_index;
    u2 signature_index;
}

The CONSTANT_Utf8 contains standard ASCII text strings. These are not null-terminated because they use an explicit length parameter. Notice that most of the constants reference other constants for information. Methods, for instance, specify a class and type by providing indexes to other constant pool members. Constant pool cross-references eliminate repetition of data.

The constant pool for the test class appears as follows:

String #28 -> Performing countdown:
Class: java/lang/System
Class: java/lang/Object
Class: test
Class: java/io/PrintStream
Field: java/lang/System.out Ljava/io/PrintStream;
Field: test.st_one I
Method: java/io/PrintStream.println(I)V
Method: java/lang/Object.<init>()V
Method: java/io/PrintStream.println(Ljava/lang/String;)V
NameAndType: println (I)V
NameAndType: println (Ljava/lang/String;)V
NameAndType: out Ljava/io/PrintStream;
NameAndType: <init> ()V
NameAndType: st_one I
Utf8: [7] println
Utf8: [4] (I)V
Utf8: [3] ()I
Utf8: [13] ConstantValue
Utf8: [4] (I)I
Utf8: [19] java/io/PrintStream
Utf8: [10] Exceptions
Utf8: [15] LineNumberTable
Utf8: [1] I
Utf8: [10] SourceFile
Utf8: [14] LocalVariables
Utf8: [4] Code
Utf8: [21] Performing countdown:
Utf8: [3] out
Utf8: [21] (Ljava/lang/String;)V
Utf8: [16] java/lang/Object
Utf8: [6] <init>
Utf8: [21] Ljava/io/PrintStream;
Utf8: [16] java/lang/System
Utf8: [12] do_countdown
Utf8: [5] ([I)Z
Utf8: [6] st_one
Utf8: [7] getData
Utf8: [9] test.java
Utf8: [3] ()V
Utf8: [4] test

Note
The ClassView tool substitutes pool indexes with actual pool data whenever possible.

Fields

Field structures contain the individual data members of a class. Any class item that is not a method is placed into the fields section. The field structure looks like the following:

field_info
{
    u2 access_flags;
    u2 name_index;
    u2 signature_index;
    u2 attribute_count;
    attribute_info attributes[attribute_count];
}

The test class contains the following field:

st_one I
  PUBLIC STATIC
  No attributes

Methods

The method section contains all of the executable content of a class. In addition to the method name and signature, the structure contains a set of attributes. One of these attributes has the actual byte codes that the virtual machine will execute. The method structure follows:

method_info
{
    u2 access_flags;
    u2 name_index;
    u2 signature_index;
    u2 attributes_count;
    attribute_info attributes[attribute_count];
}

The test class produces the following method section:

<init>()V
  PUBLIC
  Code(stack=1 locals=1 code=10 exceptions=none)
<init>(I)V
  PUBLIC
  Code(stack=1 locals=2 code=9 exceptions=none)
getData([I)Z
  PUBLIC NATIVE
  No attributes
do_countdown()I
  PUBLIC
  Code(stack=2 locals=2 code=33 exceptions=none)
do_countdown(I)I
  PUBLIC
  Code(stack=2 locals=3 code=29 exceptions=none)

Each method has a name and signature. You learned about signatures in Chapter 10, "Native Methods and Java." Each non-native method contains a code attribute that has the following format:

Code_attribute
{
    u2 attribute_name;
    u4 attribute_length;
    u2 max_stack;
    u2 max_locals;
    u4 code_length;
    u1 code[code_length];
    u2 exception_table_length;
    ExceptionItem exceptions[exception_table_length];
    u2 attributes_count;
    attribute_info attributes[attribute_count];
}

Code attributes contain a private list of other attributes. Typically, these are debugging lists, such as line number information. Now that you've hit the code attribute, it's time to jump into the virtual machine.

The Virtual Machine

The Java virtual machine interprets Java byte codes that are contained in code attributes. The virtual machine is stack based. Most computer architectures perform their operations on a mixture of memory locations and registers. The Java virtual machine performs its operations exclusively on a stack. This is done primarily to support portability. No assumptions could be made about the size or number of registers in a given CPU. Intel microprocessors are especially limited in their register composition.

Registers

The virtual machine does contain some registers, but these are used for tracking the current state of the machine:

All these registers are 32 bits wide and point into separate storage blocks. The blocks, however, can be allocated all at once because the code attribute specifies the size of the operand stack, the number of local variables, and the length of the bytecodes.

Operand Stack

Most Java byte codes work on the operand stack. For instance, to add two integers together, each integer is pushed onto the operand stack. The addition operator removes the top two integers, adds them, and places the result in its place back on the stack:

..., 4, 5 -> ..., 9

Note
The operand stack notation is used throughout the remainder of this appendix. The stack reads from left to right, with the stack top on the extreme right. Ellipses indicate indeterminate data buried on the stack. The arrow indicates an operation; the data to the right of the arrow represents the stack after the operation is performed.

Each stack location is 32 bits wide. Long and doubles are 64 bits wide, so they take up two stack locations.

Primitive Types

The virtual machine provides support for nine primitive types:

The virtual machine specification does not mandate the internal format of object references. In Sun's implementation, object references point to a Java handle consisting of two pointers. One points to the method table for the class and the other points to the object's instance data.

Local Variables

Each code attribute specifies the size of the local variables. A local variable is 32 bits wide, so long and double primitives take up two variable slots. Unlike C, all method arguments appear as local variables. The operand stack is reserved exclusively for operations.

The Verifier

When a class is loaded, it is passed through a bytecode verifier before it is executed. The verifier checks the internal consistency of the class and the validity of the code. Java uses a late binding scheme that puts the code at risk. In traditional languages, the object linker binds all of the method calls and variable accesses to specific addresses. In Java, the virtual machine doesn't perform this service until the last possible moment. As a result, it is possible for a called class to have changed since the original class was compiled. Method names or their arguments may have been altered, or the access levels may have been changed. One of the verifier's jobs is to make sure that all external object references are correct and allowed.

No assumptions can be made about the origin of bytecodes. A hostile compiler could be used to create executable bytecodes that conform to the class file format, but specify illegal codes.

The verifier uses a conservative four-pass verification algorithm to check bytecodes.

Pass 1

This pass reads in the class file and ensures that it is valid. The magic number must be present and all the class data must be present with no truncation or extra data after the end of the class. Any recognized attributes must have the correct length and the constant pool must not have any unrecognized entries.

Pass 2

The second pass involves validating class features other than the bytecodes. All methods and fields must have a valid name and signature and every class must have a super class. Signatures are not actually checked, but they must appear valid. The next pass is more specific.

Pass 3

This is the most complex pass because the bytecodes are validated. The bytecodes are analyzed to make sure that they have the correct type and number of arguments. In addition, a data-flow analysis is performed to determine each path through the method. Each path must arrive at a given point with the same stack size and types. Each path must call methods with the proper arguments, and fields must be modified with values of the appropriate type. Class accesses are not checked in this pass. Only the return type of external functions is verified.
Forcing all paths to arrive with the same stack and registers can lead the verifier to fail some otherwise legitimate bytecodes. This is a small price to pay for this high level of security.

Pass 4

This pass loads externally referenced classes and checks that the method name and signatures match. It also validates that the current class has access rights to the external class. After complete validation, each instruction is replaced with a _quick alternative. These _quick bytecodes indicate that the class has been verified and need not be checked again.

Exception Handling

The pc register points to the next bytecode to execute. Whenever an exception is thrown, the method's exception table is searched for a handler. Each exception table entry has this format:

ExceptionItem
{
    u2 start_pc;
    u2 end_pc;
    u2 handler_pc;
    u2 catch_type;
}

If the pc register is within the proper range and the thrown exception is the proper type, the entry's handler code block is executed. If no handler is found, the exception propagates up to the calling method. The procedure repeats itself until either a valid handler is found or the program exits.

Bytecodes

The bytecodes can be divided into 11 major categories:

Each bytecode has a unique tag and is followed by a fixed number of additional arguments. Notice that there is no way to work directly with class fields or local variables. They must be moved to the operand stack before any operations can be performed on the contents.

Generally, there are multiple formats for each individual operation. The addition operation provides a good example. There are actually four forms of addition: iadd, ladd, fadd, and dadd. Each type assumes the top two stack items are of the correct format: integers, longs, floats, or doubles.

Pushing Constants Onto the Stack

Java uses the following instructions for moving object data and local variables to the operand stack:

Push One-Byte Signed Integer
bipush=16 byte1      Stack: ... -> ..., byte1
Push Two-Byte Signed Integer
sipush=17 byte1 byte2    Stack: ... -> ..., word1
Push Item From the Constant Pool (8-bit index)
ldc1=18 indexbyte1    Stack: ... -> ..., item
Push Item from the Constant Pool (16-bit index)
ldc2=19 indexbyte1 indexbyte2   Stack: ... -> ..., item
Push Long or Double from Constant Pool (16-bit index)
ldc2w=20 indexbyte1 indexbyte2   Stack: ... -> ..., word1, word2
Push Null Object
aconst_null=1    Stack: ... -> ..., null
Push Integer Constant -1
iconst_m1=2   Stack: ... -> ..., -1
Push Integer Constants
iconst_0=3   Stack: ... -> ..., 0
iconst_1=4   Stack: ... -> ..., 1
iconst_2=5   Stack: ... -> ..., 2
iconst_3=6   Stack: ... -> ..., 3
iconst_4=7   Stack: ... -> ..., 4
iconst_5=8   Stack: ... -> ..., 5
Push Long Constant
lconst_0=9   Stack: ... -> ..., 0, 0
lconst_1=10  Stack: ... -> ..., 0, 1
Push Float Constants
fconst_0=11   Stack: ... -> ..., 0
fconst_1=12   Stack: ... -> ..., 1
fconst_2=13   Stack: ... -> ..., 2
Push Double Constants
dconst_0=14   Stack: ... -> ..., 0, 0
dconst_1=15   Stack: ... -> ..., 0, 1

Accessing Local Variables

The most commonly referenced local variables are at the first four offsets from the vars register. Because of this, Java provides single byte instructions to access these variables for both reading and writing. A two-byte instruction is needed to reference variables greater than 4 deep. The variable at location zero is the class pointer itself (the this pointer).

Load Integer from Local Variable
iload=21 vindex  Stack: ... -> ..., contents of varaible at vars[vindex]
iload_o=26       Stack: ... -> ..., contents of variable at vars[0]
iload_1=27       Stack: ... -> ..., contents of variable at vars[1]
iload_2=28       Stack: ... -> ..., contents of variable at vars[2]
iload_3=29       Stack: ... -> ..., contents of variable at vars[3]
Load Long Integer from Local Variable
lload=22 vindex  Stack: .. -> ..., word1, word2  from vars[vindex] & vars[vindex+1]
lload_0=30       Stack: .. -> ..., word1, word2  from vars[0] & vars[1]
lload_1=31       Stack: .. -> ..., word1, word2  from vars[1] & vars[2]
lload_2=32       Stack: .. -> ..., word1, word2  from vars[2] & vars[3]
lload_3=33       Stack: .. -> ..., word1, word2  from vars[3] & vars[4]
Load Float from Local Variable
fload=23 vindex  Stack: ... -> ..., contents from vars[vindex]
fload_0=34       Stack: ... -> ..., contents from vars[0]
fload_1=35       Stack: ... -> ..., contents from vars[1]
fload_2=36       Stack: ... -> ..., contents from vars[2]
fload_3=37       Stack: ... -> ..., contents from vars[3]
Load Double from Local Variable
dload=24 vindex  Stack: ... -> ..., word1, word2  from vars[vindex] & vars[vindex+1]
dload_0=38       Stack: ... -> ..., word1, word2  from vars[0] & vars[1]
dload_1=39       Stack: ... -> ..., word1, word2  from vars[1] & vars[2]
dload_2=40       Stack: ... -> ..., word1, word2  from vars[2] & vars[3]
dload_3=41       Stack: ... -> ..., word1, word2  from vars[3] & vars[4]
Load Object from Local Variable
aload=25 vindex  Stack: ... -> ..., object  from vars[vindex]
aload_0=42       Stack: ... -> ..., object  from vars[0]
aload_1=43       Stack: ... -> ..., object  from vars[1]
aload_2=44       Stack: ... -> ..., object  from vars[2]
aload_3=45       Stack: ... -> ..., object  from vars[3]
Store Integer into Local Variable
istore=54 vindex Stack: ..., INT -> ...  into vars[vindex]
istore_0=59      Stack: ..., INT -> ...  into vars[0]
istore_1=60      Stack: ..., INT -> ...  into vars[1]
istore_2=61      Stack: ..., INT -> ...  into vars[2]
istore_3=62      Stack: ..., INT -> ...  into vars[3]
Store Long Integer into Local Variable
lstore=55 vindex Stack: ..., word1, word2 -> ...  into vars[vindex] & vars[vindex+1]
lstore_0=63      Stack: ..., word1, word2 -> ...  into vars[0] & vars[1]
lstore_1=64      Stack: ..., word1, word2 -> ...  into vars[1] & vars[2]
lstore_2=65      Stack: ..., word1, word2 -> ...  into vars[2] & vars[3]
lstore_3=66      Stack: ..., word1, word2 -> ...  into vars[3] & vars[4]
Store Float into Local Variable
fstore=56 vindex Stack: ..., FLOAT -> ...  into vars[vindex]
fstore_0=67      Stack: ..., FLOAT -> ...  into vars[0]
fstore_1=68      Stack: ..., FLOAT -> ...  into vars[1]
fstore_2=69      Stack: ..., FLOAT -> ...  into vars[2]
fstore_3=70      Stack: ..., FLOAT -> ...  into vars[3]
Store Double into Local Variable
dstore=57 vindex Stack: ..., word1, word2 -> ...  into vars[vindex] & vars[vindex+1]
dstore_0=71      Stack: ..., word1, word2 -> ...  into vars[0] & vars[1]
dstore_1=72      Stack: ..., word1, word2 -> ...  into vars[1] & vars[2]
dstore_2=73      Stack: ..., word1, word2 -> ...  into vars[2] & vars[3]
dstore_3=74      Stack: ..., word1, word2 -> ...  into vars[3] & vars[4]
Store Object into Local Variable
istore=58 vindex Stack: ..., OBJ -> ...  into vars[vindex]
istore_0=75      Stack: ..., OBJ -> ...  into vars[0]
istore_1=76      Stack: ..., OBJ -> ...  into vars[1]
istore_2=77      Stack: ..., OBJ -> ...  into vars[2]
istore_3=78      Stack: ..., OBJ -> ...  into vars[3]
Increment Local Variable

This applies only to integers.

iinc=132 vindex constant   Stack: ... -> ...  vars[vindex] += constant

Managing Arrays

Allocate a New Array
newarray=188 type Stack: ..., size -> ..., OBJ
Allocate a New Array of Objects
anewarray=189 classindex1 classindex2 Stack: ..., size -> ..., OBJ
Allocate a New Multi-Dimensional Array
newarray=197 indexbyte1 indexbyte1 indexbyte2  Stack: ..., size1, size2, etc -> ..., OBJ
Get the Array Length
arraylength=190 Stack: ..., OBJ -> ..., length
Load Primitives from the Array
iaload=46 Stack: ..., OBJ, index -> ..., INT
laload=47 Stack: ..., OBJ, index -> ..., LONG1, LONG2
faload=48 Stack: ..., OBJ, index -> ..., FLOAT
daload=49 Stack: ..., OBJ, index -> ..., DOUBLE1, DOUBLE2
aaload=50 Stack: ..., OBJ, index -> ..., OBJ
baload=51 Stack: ..., OBJ, index -> ..., BYTE
caload=52 Stack: ..., OBJ, index -> ..., chAR
saload=53 Stack: ..., OBJ, index -> ..., SHORT
Store Primitives into the Array
iastore-79 Stack: ..., OBJ, index, INT -> ...
lastore=80 Stack: ..., OBJ, index, LONG1, LONG2 -> ...
fastore=81 Stack: ..., OBJ, index, FLOAT -> ...
dastore=82 Stack: ..., OBJ, index, DOUBLE1, DOUBLE2 -> ...
aastore=83 Stack: ..., OBJ, index, OBJ -> ...
bastore=84 Stack: ..., OBJ, index, BYTE -> ...
castore=85 Stack: ..., OBJ, index, chAR -> ...
sastore=86 Stack: ..., OBJ, index, SHORT -> ...

Generic Stack Instructions

These are basic operations that alter the stack:

Do Nothing
nop=0   Stack: ... -> ...
Pop Stack Values
pop=87  Stack: ..., VAL -> ...
pop2=88  Stack: ..., VAL1, VAL2 -> ...
Duplicate Stack Values and Possibly Insert Below Stack Top
dup=89     Stack: ..., V -> ..., V, V
dup2=92    Stack: ..., V1, V2 -> ..., V1, V2, V1, V2
dup_x1=90  Stack: ..., V1, V2 -> ..., V2, V1, V2
dup2_x1=93 Stack: ..., V1, V2, V3 -> ..., V2, V3, V1, V2, V3
dup_x2=91  Stack: ..., V1, V2, V3 -> ..., V3, V1, V2, V3
dup2_x2=94 Stack: ..., V1, V2, V3, V4 -> ..., V3, V4, V1, V2, V3, V4
Swap Two-Stack Items
swap=95    Stack: ..., V1, V2 -> ..., V2, V1

Arithmetic and Logical Instructions

All the arithmetic operations operate on four possible types: integer, long, float, or double. Logical instructions operate only on integer and long types.

Addition
iadd=96  Stack: ..., INT1, INT2 -> ..., INT1+INT2
ladd=97  Stack: ..., L1_1, L1_2, L2_1, L2_2 -> ..., L1+L2 (high), L1+L2 (low)
fadd=98  Stack: ..., FLOAT1, FLOAT2 -> ..., FLOAT1+FLOAT2
dadd=99  Stack: ..., D1_1, D1_2, D2_1, D2_2 -> ..., D1+D2 (high), D1+D2 (low)
Subtraction
isub=100 Stack: ..., INT1, INT2 -> ..., INT1-INT2
lsub=101 Stack: ..., L1_1, L1_2, L2_1, L2_2 -> ..., L1-L2 (high), L1-L2 (low)
fsub=102 Stack: ..., FLOAT1, FLOAT2 -> ..., FLOAT1-FLOAT2
dsub=103 Stack: ..., D1_1, D1_2, D2_1, D2_2 -> ..., D1-D2 (high), D1-D2 (low)
Multiplication
imul=104  Stack: ..., INT1, INT2 -> ..., INT1*INT2
lmul=105  Stack: ..., L1_1, L1_2, L2_1, L2_2 -> ..., L1*L2 (high), L1*L2 (low)
fmul=106  Stack: ..., FLOAT1, FLOAT2 -> ..., FLOAT1*FLOAT2
dmul=107  Stack: ..., D1_1, D1_2, D2_1, D2_2 -> ..., D1*D2 (high), D1*D2 (low)
Division
idiv=108  Stack: ..., INT1, INT2 -> ..., INT1/INT2
ldiv=109  Stack: ..., L1_1, L1_2, L2_1, L2_2 -> ..., L1/L2 (high), L1/L2 (low)
fdiv=110  Stack: ..., FLOAT1, FLOAT2 -> ..., FLOAT1/FLOAT2
ddiv=111  Stack: ..., D1_1, D1_2, D2_1, D2_2 -> ..., D1/D2 (high), D1/D2 (low)
Remainder
irem=112  Stack: ..., INT1, INT2 -> ..., INT1%INT2
lrem=113  Stack: ..., L1_1, L1_2, L2_1, L2_2 -> ..., L1%L2 (high), L1%L2 (low)
frem=114  Stack: ..., FLOAT1, FLOAT2 -> ..., FLOAT1%FLOAT2
drem=115  Stack: ..., D1_1, D1_2, D2_1, D2_2 -> ..., D1%D2 (high), D1%D2 (low)
Negation
ineg=116  Stack: ..., INT -> ..., -INT
lneg=117  Stack: ..., LONG1, LONG2 -> ..., -LONG1, -LONG2
fneg=118  Stack: ..., FLOAT -> ..., -FLOAT
dneg=119  Stack: ..., DOUBLE1, DOUBLE2 -> ..., -DOUBLE1, -DOUBLE2
Integer Logical Instructions

>>> denotes an unsigned right shift.

ishl=120  Stack: ..., INT1, INT2 -> INT1<<(INT2 & 0x1f)
ishr=122  Stack: ..., INT1, INT2 -> INT1>>(INT2 & 0x1f)
iushr=124 Stack: ..., INT1, INT2 -> INT1>>>(INT2 & 0x1f)
Long Integer Logical Instructions

>>> denotes an unsigned right shift.

lshl=121  Stack: ..., L1, L2, INT -> L1<<(INT & 0x3f), L2<<(INT & 0x3f)
lshr=123  Stack: ..., L1, L2, INT -> INT1>>(INT & 0x3f), L2>>(INT & 0x03)
lushr=125 Stack: ..., L1, L2, INT -> INT1>>>(INT & 0x3f), L2>>>(INT & 0x3f)
Integer Boolean Operations
iand=126  Stack: ..., INT1, INT2 -> ..., INT1&INT2
ior=128   Stack: ..., INT1, INT2 -> ..., INT1|INT2
ixor=130  Stack: ..., INT1, INT2 -> ..., INT1^INT2
Long Integer Boolean Operations
land=127  Stack: ..., L1_1, L1_2, L2_1, L2_2 -> ..., L1_1&L2_1, L1_2&L2_2
lor=129   Stack: ..., L1_1, L1_2, L2_1, L2_2 -> ..., L1_1|L2_1, L1_2|L2_2
lxor=131  Stack: ..., L1_1, L1_2, L2_1, L2_2 -> ..., L1_1^L2_1. L1_2^L2_2

Conversion Instructions

Because most of the previous bytecodes expect the stack to contain a homogenous set of operands, Java uses conversion functions. In code, you can add a float and an integer, but Java will first convert the integer to a float type before performing the addition.

Integer Conversions
i2l=133       Stack: .., INT -> ..., LONG1, LONG2
i2f=134       Stack: .., INT -> ..., FLOAT
i2d=135       Stack: .., INT -> ..., DOUBLE1, DOUBLE2
int2byte=145  Stack: .., INT -> ..., BYTE
int2char=146  Stack: .., INT -> ..., chAR
int2short=147 Stack: .., INT -> ..., SHORT
Long Integer Conversions
l2i=136       Stack: .., LONG1, LONG2 -> ..., INT
l2f=137       Stack: .., LONG1, LONG2 -> ..., FLOAT
l2d=138       Stack: .., LONG1, LONG2 -> ..., DOUBLE1, DOUBLE2
Float Conversions
f2i=139       Stack: .., FLOAT -> ..., INT
f2l=140       Stack: .., FLOAT -> ..., LONG1, LONG2
f2d=141       Stack: .., FLOAT -> ..., DOUBLE1, DOUBLE2
Double Conversions
d2i=142       Stack: .., DOUBLE1, DOUBLE2 -> ..., INT
d2l=143       Stack: .., DOUBLE1, DOUBLE2 -> ..., LONG1, LONG2
d2f=144       Stack: .., DOUBLE1, DOUBLE2 -> ..., FLOAT

Control Transfer and Function Return

All branch indexes are signed 16-bit offsets from the current pc register.

Comparisons with Zero
ifeq=153 branch1 branch2   Stack: ..., INT -> ...
ifne=154 branch1 branch2   Stack: ..., INT -> ...
iflt=155 branch1 branch2   Stack: ..., INT -> ...
ifge=156 branch1 branch2   Stack: ..., INT -> ...
ifgt=157 branch1 branch2   Stack: ..., INT -> ...
ifle=158 branch1 branch2   Stack: ..., INT -> ...
Comparison with Null
ifnull=198 branch1 branch2    Stack: ..., OBJ -> ...
ifnonnull=199 branch1 branch2 Stack: ..., OBJ -> ...
Compare Two Integers
if_icmpeq=159 branch1 branch2  Stack: ..., INT1, INT2 -> ...
if_icmpne=160 branch1 branch2  Stack: ..., INT1, INT2 -> ...
if_icmplt=161 branch1 branch2  Stack: ..., INT1, INT2 -> ...
if_icmpge=162 branch1 branch2  Stack: ..., INT1, INT2 -> ...
if_icmpgt=163 branch1 branch2  Stack: ..., INT1, INT2 -> ...
if_icmple=164 branch1 branch2  Stack: ..., INT1, INT2 -> ...
Compare Two Long Integers
lcmp=148 Stack: ..., L1_1, L1_2, L2_1, L2_2 -> ..., INT (One of [-1, 0, 1])
Compare Two Floats

l->-1 on NaN, g->1 on NaN.

fcmpl=149  Stack: ..., FLOAT1, FLOAT2 -> ..., INT (One of [-1, 0, 1])
fcmpg=150  Stack: ..., FLOAT1, FLOAT2 -> ..., INT (One of [-1, 0, 1])
Compare Two Doubles

l->-1 on NaN, g->1 on NaN.

dcmpl=151  Stack: ..., D1_1, D1_2, D2_1, D2_2 -> ..., INT (One of [-1, 0, 1])
dcmpg=152  Stack: ..., D1_1, D1_2, D2_1, D2_2 -> ..., INT (One of [-1, 0, 1])
Compare Two Objects
if_acmpeq=165 branch1 branch2  Stack: ..., OBJ1, OBJ2 -> ...
if_acmpne=166 branch1 branch2  Stack: ..., OBJ1, OBJ2 -> ...
Unconditional Branching

16-bit and 32-bit branching

goto=167 branch1 branch2                    Stack: ... -> ...
goto_w=200 branch1 branch2 branch3 branch4  Stack: ... -> ...
Jump Subroutine

16-bit and 32-bit jumps

jsr=168 branch1 branch2                     Stack: ... -> ..., returnAddress
jsr_w=201 branch1 branch2 branch3 branch4   Stack: ... -> ..., returnAddress
Return from Subroutine

The return address is retrieved from a local variable, not the stack.

ret=169 vindex   Stack: ... -> ...   (returnAddress <- vars[vindex])
ret_w=209 vindex1 vindex2   Stack: ... -> ...   (returnAddress <- vars[vindex])
Returning Primitives

The current stack frame is destroyed. The top primitive is pushed onto the caller's operand stack.

ireturn=172  Stack: ..., INT -> [destroyed]
lreturn=173  Stack: ..., LONG1, LONG2 -> [destroyed]
freturn=174  Stack: ..., FLOAT -> [destroyed]
dreturn=175  Stack: ..., DOUBLE1, DOUBLE2 -> [destroyed]
areturn=176  Stack: ..., OBJ -> [destroyed]
return=177  Stack: ... -> [destroyed]
Calling the Breakpoint Handler
breakpoint=202  Stack: ..., -> ...

Manipulating Object Fields

Construct a 16-bit index into the constant pool to retrieve the class and field name, then resolve these names to determine the field offset and width. Use the object reference on the stack as the target. The value will be 32 or 64 bits, depending on the field information in the constant pool.

Getstatic=178  index1 index2   Stack: ..., -> ..., VAL
Putstatic=179  index1 index2   Stack: ..., VAL -> ...
Getfield=180   index1 index2   Stack: ..., OBJ -> ..., VAL
Putfield=181   index1 index2   Stack: ..., OBJ, VAL -> ...

Method Invocation

There are four types of method invocation:

Miscellaneous Operations

These instructions don't fall under any other heading; they deal with generic object operations, such as creation and casting.

Throw Exception
athrow=191  Stack: ..., OBJ -> [undefined]
Create a New Object
new=187 index1 index2  Stack: ... -> ..., OBJ
Check a Cast Operation
checkcast=192 index1 index2  Stack: ..., OBJ -> ..., OBJ
Instanceof
instanceof=193 index1 index2  Stack: ..., OBJ -> ... INT (1 or 0)

Monitors

Monitor instructions are used for synchronization.

Enter a Monitored Region of Code
monitorenter=194  Stack: ..., OBJ -> ...
Exit a Monitored Region of Code
monitorexit=195  Stack: ..., OBJ -> ...

Test Class Bytecodes

Sun supplies a tool, javap, that enables you to disassemble and view the bytecodes of a class. If the -c option is passed to javap, a listing of bytecodes is produced. These are the test class's bytecodes:

Compiled from test.java
public class test extends java.lang.Object {
    public static int st_one;
    public test();
    public test(int);
    public native boolean getData(int []);
    public int do_countdown();
    public int do_countdown(int);

Method test()
   0 aload_0
   1 invokenonvirtual #9 <Method java.lang.Object.<init>()V>
   4 bipush 100
   6 putstatic #7 <Field test.st_one I>
   9 return

Method test(int)
   0 aload_0
   1 invokenonvirtual #9 <Method java.lang.Object.<init>()V>
   4 iload_1
   5 putstatic #7 <Field test.st_one I>
   8 return

Method int do_countdown()
   0 getstatic #7 <Field test.st_one I>
   3 istore_1
   4 getstatic #6 <Field java.lang.System.out Ljava/io/PrintStream;>
   7 ldc #1 <String "Performing countdown:">
   9 invokevirtual #10 <Method java.io.PrintStream.println(Ljava/lang/String;)V>
  12 goto 22
  15 getstatic #6 <Field java.lang.System.out Ljava/io/PrintStream;>
  18 iload_1
  19 invokevirtual #8 <Method java.io.PrintStream.println(I)V>
  22 iload_1
  23 iinc 1 -1
  26 ifne 15
  29 getstatic #7 <Field test.st_one I>
  32 ireturn

Method int do_countdown(int)
   0 iload_1
   1 istore_2
   2 getstatic #6 <Field java.lang.System.out Ljava/io/PrintStream;>
   5 ldc #1 <String "Performing countdown:">
   7 invokevirtual #10 <Method java.io.PrintStream.println(Ljava/lang/String;)V>
  10 goto 20
  13 getstatic #6 <Field java.lang.System.out Ljava/io/PrintStream;>
  16 iload_1
  17 invokevirtual #8 <Method java.io.PrintStream.println(I)V>
  20 iload_1
  21 iinc 1 -1
  24 ifne 13
  27 iload_2
  28 ireturn
}

The left-hand column displays the offset of the instruction. Javap automatically converts jump displacements to actual offsets. In addition, it looks up constant pool references in order to output the corresponding strings.

Garbage Collection

Java uses a multitiered security mechanism. The bytecode verifier provides the lowest layer of security. Above the verifier, the security manager is the next sentry. In addition to these two explicit checks, there are a number of language features that provide security as well. Chief among these is the garbage collector.

Failing to free memory blocks or file handles is a common bug in most modern programs. The problem quickly escalates until the system crashes in some unforeseen manner. Java, like Smalltalk before it, uses implicit garbage collection to solve the problem. The virtual machine spec does not mandate a particular type of garbage collection; it requires only that some type be used.

In Sun's runtime, a mark and sweep algorithm is used. This enables the garbage collector to run incrementally in the background.