What is a Java Virtual Machine?
The Java virtual machine is called “virtual” because it is an abstract computer defined by a specification. To run a Java program, you need a concrete implementation of the abstract specification. This blog describes primarily the abstract specification of the Java virtual machine.
The Lifetime of a Java Virtual Machine
A runtime instance of the Java virtual machine has a clear mission in life: to run one Java application. When a Java application starts, a runtime instance is born. When the application completes, the instance dies. If you start three Java applications at the same time, on the same computer, using the same concrete implementation, you’ll get three Java virtual machine instances. Each Java application runs inside its own Java virtual machine.
A Java virtual machine instance starts running its solitary application by invoking the main() method of some initial class. The main() method must be public, static, return void, and accept one parameter: a String array. Any class with such a main() method can be used as the starting point for a Java application.
For example, consider an application that prints out its command line arguments:
Code:
// On CD-ROM in file jvm/ex1/Echo.javaclass Echo {public static void main(String[] args) {int len = args.length;for (int i = 0; i < len; ++i) {System.out.print(args[i] + " ");}System.out.println();}}
You must in some implementation-dependent way give a Java virtual machine the name of the initial class that has the main() method that will start the entire application. One real world example of a Java virtual machine implementation is the java program from Sun’s Java 2 SDK. If you wanted to run the Echo application using Sun’s java on Window98, for example, you would type in a command such as:
Java Echo Greetings, Planet.
The first word in the command, “java,” indicates that the Java virtual machine from Sun’s Java 2 SDK should be run by the operating system. The second word, “Echo,” is the name of the initial class. Echo must have a public static method named main() that returns void and takes a String array as its only parameter. The subsequent words, “Greetings, Planet.,” are the command line arguments for the application. These are passed to the main() method in the String array in the order in which they appear on the command line. So, for the previous example, the contents of the String array passed to main in Echo are: arg[0] is “Greetings,” arg[1] is “Planet.”
The main() method of an application’s initial class serves as the starting point for that application’s initial thread. The initial thread can in turn fire off other threads.
Inside the Java virtual machine, threads come in two flavors: daemon and non- daemon. A daemon thread is ordinarily a thread used by the virtual machine itself, such as a thread that performs garbage collection. The application, however, can mark any threads it creates as daemon threads. The initial thread of an application–the one that begins at main()–is a non- daemon thread.
A Java application continues to execute (the virtual machine instance continues to live) as long as any non-daemon threads are still running. When all non-daemon threads of a Java application terminate, the virtual machine instance will exit. If permitted by the security manager, the application can also cause its own demise by invoking the exit() method of class Runtime or System.
In the Echo application previous, the main() method doesn’t invoke any other threads. After it prints out the command line arguments, main() returns. This terminates the application’s only non-daemon thread, which causes the virtual machine instance to exit.
The Architecture of the Java Virtual Machine
In the Java virtual machine specification, the behavior of a virtual machine instance is described in terms of subsystems, memory areas, data types, and instructions. These components describe an abstract inner architecture for the abstract Java virtual machine. The purpose of these components is not so much to dictate an inner architecture for implementations. It is more to provide a way to strictly define the external behavior of implementations. The specification defines the required behavior of any Java virtual machine implementation in terms of these abstract components and their interactions.
The block diagram of the Java virtual machine that includes the major subsystems and memory areas described in the specification . Each Java virtual machine has a class loader subsystem: a mechanism for loading types (classes and interfaces) given fully qualified names. Each Java virtual machine also has an execution engine: a mechanism responsible for executing the instructions contained in the methods of loaded classes.
When a Java virtual machine runs a program, it needs memory to store many things, including byte codes and other information it extracts from loaded class files, objects the program instantiates, parameters to methods, return values, local variables, and intermediate results of computations. The Java virtual machine organizes the memory it needs to execute a program into several runtime data areas.
Although the same runtime data areas exist in some form in every Java virtual machine implementation, their specification is quite abstract. Many decisions about the structural details of the runtime data areas are left to the designers of individual implementations.
Different implementations of the virtual machine can have very different memory constraints. Some implementations may have a lot of memory in which to work, others may have very little. Some implementations may be able to take advantage of virtual memory, others may not. The abstract nature of the specification of the runtime data areas helps make it easier to implement the Java virtual machine on a wide variety of computers and devices.
Some runtime data areas are shared among all of an application’s threads and others are unique to individual threads. Each instance of the Java virtual machine has one method area and one heap. These areas are shared by all threads running inside the virtual machine. When the virtual machine loads a class file, it parses information about a type from the binary data contained in the class file. It places this type information into the method area. As the program runs, the virtual machine places all objects the program instantiates onto the heap .
As each new thread comes into existence, it gets its own pc register (program counter) and Java stack. If the thread is executing a Java method (not a native method), the value of the pc register indicates the next instruction to execute. A thread’s Java stack stores the state of Java (not native) method invocations for the thread. The state of a Java method invocation includes its local variables, the parameters with which it was invoked, its return value (if any), and intermediate calculations. The state of native method invocations is stored in an implementation-dependent way in native method stacks, as well as possibly in registers or other implementation-dependent memory areas.
The Java stack is composed of stack frames (or frames). A stack frame contains the state of one Java method invocation. When a thread invokes a method, the Java virtual machine pushes a new frame onto that thread’s Java stack. When the method completes, the virtual machine pops and discards the frame for that method.
The Java virtual machine has no registers to hold intermediate data values. The instruction set uses the Java stack for storage of intermediate data values. This approach was taken by Java’s designers to keep the Java virtual machine’s instruction set compact and to facilitate implementation on architectures with few or irregular general purpose registers. In addition, the stack-based architecture of the Java virtual machine’s instruction set facilitates the code optimization work done by just-in-time and dynamic compilers that operate at run-time in some virtual machine implementations.
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