Class Files and Byte Code in Java

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When Java source code is compiled, the results of the compilation are put into Java class files, whose names typically end with the .class or .cls extension. Java class files are made up of streams of 8-bit bytes. Larger values requiring 16 or 32 bits are composed of multiple 8-bit bytes. Class files contain several pieces of information in a particular format. Included in a class file are:
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The magic constant (0xCAFEBABE) Major and minor version information The constant pool (a heterogeneous array composed of five primitive types) Information about the class (name, superclass, etc.) Information about interfaces Information about the fields and methods in the class Debugging information
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[Sun Microsystems, 1996b; Sun Microsystems, 1996c] Much more information on class file formats and byte code syntax can be found in [Venners, 1998]. The byte code inside a class file is made up of instructions that can be divided into several categories. Among other things, byte code instructions, called opcodes, implement:
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Pushing constants onto the stack Accessing and modifying the value of a VM register Accessing arrays Manipulating the stack Arithmetic instructions Logic instructions Conversion instructions Control transfer Function return Manipulating object fields Invoking methods Creating objects Type casting
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Since it exists at the level of the VM, Java byte code is very similar to assembly language. Each line of
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byte code is a one-byte opcode followed by zero or more bytes of operand information. All instructions (with the exception of two table lookup instructions) are of fixed length. Opcodes and their associated operands represent the fundamental operations of the VM. Every method invocation in Java gets its own stack frame to use as local storage for variable values and intermediate results. As discussed earlier, the intermediate storage area part of a frame is called the operand stack. Opcodes refer to data stored either on the operand stack or in the local variables of a method's frame. The VM uses these values as well as direct operand values from an instruction as execution data.
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Internal Checks Considered one level above the nitty-gritty level, class verification (of which byte code verification is a crucial step) is usually said to occur in four passes. The first three passes implement the Verifier's internal checks. The last pass implements the runtime checks. (Why runtime checking is considered a part of the load-time verification process is a mystery to us, but Sun seems to have convinced everyone to call it a fourth verification pass, so we'll go along.) The passes are: 1. Ensure that the class file is in the proper format. This includes checking the magic number and making sure that all attributes are of the right length. The byte code cannot be too short or too long, and the constant pool is able to be parsed. 2. Verify anything that can be done without looking at the opcodes. This includes the following checks: 1. final classes cannot be subclassed, and final methods cannot be overridden. 2. Every class must have a superclass (except the class java.lang.Object). 3. The constant pool must satisfy more stringent constraints. 4. All field references and method references in the constant pool must have legal names, legal classes, and a legal type signature. 3. Verify the byte code using data-flow analysis. At any given point in the byte code program, no matter how that point is reached, all of the following must hold: 1. The operand stack is always the same size and includes the same types. 2. Register access is checked for proper value type. 3. Methods are called with the appropriate number and types of arguments. 4. Fields are modified with values of the appropriate type. 5. All opcodes have proper type arguments on the stack and in the registers. 6. Variables are properly initialized. (See [Sun Microsystems, 1996c].) 4. Perform at runtime any checks that were not done at verification time. Some of these checks might have been impossible at verification time since some aspects of Java's type system cannot be statically checked, and some checks might have been deferred to runtime for implementation convenience. Step 3, in which the actual byte code is verified, is a complex process that is carried out in two passes by Sun's Verifier. (Other vendors' Verifiers may behave differently.) The first pass identifies individual opcodes and stores them in a table. Once all opcodes are identified, the second pass parses each opcode's
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operands. During the second pass, a structure is built for each byte code instruction. This structure is evaluated for syntactic correctness by checking that:
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