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JAVA VIRTUAL MACHINE
What is the Java Virtual Machine? Why is it here?
The Java Virtual Machine, or JVM, is an abstract computer that runs compiled Java programs. The JVM is "virtual" because it is generally implemented in software on top of a "real" hardware platform and operating system. All Java programs are compiled for the JVM. Therefore, the JVM must be implemented on a particular platform before compiled Java programs will run on that platform.
The JVM plays a central role in making Java portable. It provides a layer of abstraction between the compiled Java program and the underlying hardware platform and operating system. The JVM is central to Java's portability because compiled Java programs run on the JVM, independent of whatever may be underneath a particular JVM implementation. What makes the JVM lean and mean? The JVM is lean because it is small when implemented in software. It was designed to be small so that it can fit in as many places as possible places like TV sets, cell phones, and personal computers. The JVM is mean because it of its ambition. "Ubiquity!" is its battle cry. It wants to be everywhere, and its success is indicated by the extent to which programs written in Java will run everywhere.
Java bytecodes
Java programs are compiled into a form called Java bytecodes. The JVM executes Java bytecodes, so Java bytecodes can be thought of as the machine language of the JVM. The Java compiler reads Java language source (.java) files, translates the source into Java bytecodes, and places the bytecodes into class (.class) files. The compiler generates one class file per class in the source.
To the JVM, a stream of bytecodes is a sequence of instructions. Each instruction consists of a onebyte opcode and zero or more operands. The opcode tells the JVM what action to take. If the JVM requires more information to perform the action than just the opcode, the required information immediately follows the opcode as operands. A mnemonic is defined for each bytecode instruction. The mnemonics can be thought of as an assembly language
JAVA VIRTUAL MACHINE
for the JVM. For example, there is an instruction that will cause the JVM to push a zero onto the stack. The mnemonic for this instruction is iconst_0, and its bytecode value is 60 hex. This instruction takes no operands. Another instruction causes program execution to unconditionally jump forward or backward in memory. This instruction requires one operand, a 16bit signed offset from the current memory location. By adding the offset to the current memory location, the JVM can determine the memory location to jump to. The mnemonic for this instruction is goto, and its bytecode value is a7 hex.
Virtual parts
The "virtual hardware" of the Java Virtual Machine can be divided into four basic parts: the registers, the stack, the garbagecollected heap, and the method area. These parts are abstract, just like the machine they compose, but they must exist in some form in every JVM implementation. The size of an address in the JVM is 32 bits.The JVM can, therefore, address up to 4 gigabytes (2 to the power of 32) of memory, with each memory location containing one byte. Each register in the JVM stores one 32bit address. The stack, the garbagecollected heap, and the method area reside somewhere within the 4 gigabytes of addressable memory. The exact location of these memory areas is a decision of the implementor of each particular JVM. A word in the Java Virtual Machine is 32 bits. The JVM has a small number of primitive data types: byte (8 bits), short (16 bits), int (32 bits), long (64 bits), float (32 bits), double (64 bits), and char (16 bits). With the exception of char, which is an unsigned Unicode character, all the numeric types are signed. These types conveniently map to the types available to the Java programmer. One other primitive type is the object handle, which is a 32bit address that refers to an object on the heap. The method area, because it contains bytecodes, is aligned on byte boundaries. The stack and garbagecollected heap are aligned on word (32bit) boundaries.
The proud, the few, the registers
The JVM has a program counter and three registers that manage the stack. It has few registers because the bytecode instructions of the JVM operate primarily on the stack. This stackoriented design helps keep the JVM's instruction set and implementation small. The JVM uses the program counter, or pc register, to keep track of where in memory it should be executing instructions. The other three registers optop register, frame register, and vars register point to various parts of the stack frame of the currently executing method. The stack frame of an executing method holds the state (local variables, intermediate results of calculations, etc.) for a particular invocation of the method.
The method area and the program counter
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The method area is where the bytecodes reside. The program counter always points to (contains the address of) some byte in the method area. The program counter is used to keep track of the thread of execution. After a bytecode instruction has been executed, the program counter will contain the address of the next instruction to execute. After execution of an instruction, the JVM sets the program counter to the address of the instruction that immediately follows the previous one, unless the previous one specifically demanded a jump.
The Java stack and related registers
The Java stack is used to store parameters for and results of bytecode instructions, to pass parameters to and return values from methods, and to keep the state of each method invocation. The state of a method invocation is called its stack frame. The vars, frame, and optop registers point to different parts of the current stack frame. There are three sections in a Java stack frame: the local variables, the execution environment, and the operand stack. The local variables section contains all the local variables being used by the current method invocation. It is pointed to by the vars register. The execution environment section is used to maintain the operations of the stack itself. It is pointed to by the frame register. The operand stack is used as a work space by bytecode instructions. It is here that the parameters for bytecode instructions are placed, and results of bytecode instructions are found. The top of the operand stack is pointed to by the optop register. The execution environment is usually sandwiched between the local variables and the operand stack. The operand stack of the currently executing method is always the topmost stack section, and the optop register therefore always points to the top of the entire Java stack.
The garbagecollected heap
The heap is where the objects of a Java program live. Any time you allocate memory with the new operator, that memory comes from the heap. The Java language doesn't allow you to free allocated memory directly. Instead, the runtime environment keeps track of the references to each object on the heap, and automatically frees the memory occupied by objects that are no longer referenced a process called garbage collection.
Eternal math: a JVM simulation
The applet below simulates a JVM executing a few bytecode instructions. The instructions in the simulation were generated by the javac compiler given the following java code: class Act { public static void doMathForever() { int i = 0; while (true) { i += 1;
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i *= 2; } } } The instructions in the simulation represent the body of the doMathForever() method. These instructions were chosen because they are a short sequence of bytecodes that do something mildly interesting on the stack. This simulation stars the registers, the stack, and the method area. The heap is not involved in this bytecode sequence, so it is not shown as part of the applet's user interface. All numbers in the simulation are shown in hex. As our story opens, the program counter (pc register) is pointing to an iconst_0 instruction. The iconst_0 instruction is in the method area, where bytecodes like to hang out. When you press the Step button, the JVM will execute the single instruction that is being pointed to by the program counter. So, the first time you press the Step button, the iconst_0 instruction, which pushes a zero onto the stack, will be executed. After this instruction has executed,the program counter will be pointing to the next instruction to execute.Subsequent presses of the Step button will execute subsequent instructions and the program counter will lead the way. Pressing the Reset button will cause the simulation to start over at the beginning. The value of each register is shown two ways. The contents of each register, a 32bit address, is shown in hex across the top of the simulation. Additionally, I put a small pointer to the address contained in each register next to the address in either the stack or the method area. The address contained by the program counter, for example, has a pc> next to it in the method area.
ADVANTAGES OF JVM A selfcontained operating environment that behaves as if it is a separate computer. For example, Java applets run in a Java virtual machine (VM) that has no access to the host operating system. This design has two
advantages:
• •
System Independence: A Java application will run the same in any Java VM, regardless of the hardware and
software underlying the system.
Security: Because the VM has no contact with the operating system, there is little possibility of a Java program damaging other files or applications.
The second advantage, however, has a downside. Because programs running in a VM are separate from the operating system, they cannot take advantage of special operating system features.
WHY GARBAGE COLLECTION
The Java virtual machine's heap stores all objects created by a running Java application. Objects are created by the new, newarray, anewarray, and multianewarray instructions, but never freed explicitly by the code. Garbage collection is the process of automatically freeing objects that are no longer referenced by the program.
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This chapter does not describe an official Java garbagecollected heap, because none exists. As mentioned in earlier chapters, the Java virtual machine specification does not require any particular garbage collection technique. It doesn't even require garbage collection at all. But until infinite memory is invented, most Java virtual machine implementations will likely come with garbagecollected heaps. This chapter describes various garbage collection techniques and explains how garbage collection works in Java virtual machines. Accompanying this chapter on the CDROM is an applet that interactively illustrates the material presented in the chapter. The applet, named Heap of Fish, simulates a garbagecollected heap in a Java virtual machine. The simulation which demonstrates a compacting, markandsweep collectorallows you to interact with the heap as if you were a Java program: you can allocate objects and assign references to variables. The simulation also allows you to interact with the heap as if you were the Java virtual machine: you can drive the processes of garbage collection and heap compaction. At the end of this chapter, you will find a description of this applet and instructions on how to use it.
Why Garbage Collection?
The name "garbage collection" implies that objects no longer needed by the program are "garbage" and can be thrown away. A more accurate and uptodate metaphor might be "memory recycling." When an object is no longer referenced by the program, the heap space it occupies can be recycled so that the space is made available for subsequent new objects. The garbage collector must somehow determine which objects are no longer referenced by the program and make available the heap space occupied by such unreferenced objects. In the process of freeing unreferenced objects, the garbage collector must run any finalizers of objects being freed. In addition to freeing unreferenced objects, a garbage collector may also combat heap fragmentation. Heap fragmentation occurs through the course of normal program execution. New objects are allocated, and unreferenced objects are freed such that free portions of heap memory are left in between portions occupied by live objects. Requests to allocate new objects may have to be filled by extending the size of the heap even though there is enough total unused space in the existing heap. This will happen if there is not enough contiguous free heap space available into which the new object will fit. On a virtual memory system, the extra paging (or swapping) required to service an ever growing heap can degrade the performance of the executing program. On an embedded system with low memory, fragmentation could cause the virtual machine to "run out of memory" unnecessarily. Garbage collection relieves you from the burden of freeing allocated memory. Knowing when to explicitly free allocated memory can be very tricky. Giving this job to the Java virtual machine has several advantages. First, it can make you more productive. When programming in nongarbagecollected languages you can spend many late hours (or days or weeks) chasing down an elusive memory problem. When programming in Java you can use that time more advantageously by getting ahead of schedule or simply going home to have a life. A second advantage of garbage collection is that it helps ensure program integrity. Garbage collection is an
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important part of Java's security strategy. Java programmers are unable to accidentally (or purposely) crash the Java virtual machine by incorrectly freeing memory. A potential disadvantage of a garbagecollected heap is that it adds an overhead that can affect program performance. The Java virtual machine has to keep track of which objects are being referenced by the executing program, and finalize and free unreferenced objects on the fly. This activity will likely require more CPU time than would have been required if the program explicitly freed unnecessary memory. In addition, programmers in a garbagecollected environment have less control over the scheduling of CPU time devoted to freeing objects that are no longer needed.