Native Method Stacks

In addition to all the runtime data areas defined by the Java virtual machine specification and described previously, a running Java application may use other data areas created by or for native methods. When a thread invokes a native method, it enters a new world in which the structures and security restrictions of the Java virtual machine no longer hamper its freedom. A native method can likely access the runtime data areas of the virtual machine (it depends upon the native method interface), but can also do anything else it wants. It may use registers inside the native processor, allocate memory on any number of native heaps, or use any kind of stack.

Native methods are inherently implementation dependent. Implementation designers are free to decide what mechanisms they will use to enable a Java application running on their implementation to invoke native methods.

Any native method interface will use some kind of native method stack. When a thread invokes a Java method, the virtual machine creates a new frame and pushes it onto the Java stack. When a thread invokes a native method, however, that thread leaves the Java stack behind. Instead of pushing a new frame onto the thread's Java stack, the Java virtual machine will simply dynamically link to and directly invoke the native method. One way to think of it is that the Java virtual machine is dynamically extending itself with native code. It is as if the Java virtual machine implementation is just calling another (dynamically linked) method within itself, at the behest of the running Java program.

If an implementation's native method interface uses a C-linkage model, then the native method stacks are C stacks. When a C program invokes a C function, the stack operates in a certain way. The arguments to the function are pushed onto the stack in a certain order. The return value is passed back to the invoking function in a certain way. This would be the behavior of the of native method stacks in that implementation.

A native method interface will likely (once again, it is up to the designers to decide) be able to call back into the Java virtual machine and invoke a Java method. In this case, the thread leaves the native method stack and enters another Java stack.

Figure 5-13 shows a graphical depiction of a thread that invokes a native method that calls back into the virtual machine to invoke another Java method. This figure shows the full picture of what a thread can expect inside the Java virtual machine. A thread may spend its entire lifetime executing Java methods, working with frames on its Java stack. Or, it may jump back and forth between the Java stack and native method stacks.

Figure 5-13. The stack for a thread that invokes Java and native methods.

As depicted in Figure 5-13, a thread first invoked two Java methods, the second of which invoked a native method. This act caused the virtual machine to use a native method stack. In this figure, the native method stack is shown as a finite amount of contiguous memory space. Assume it is a C stack. The stack area used by each C-linkage function is shown in gray and bounded by a dashed line. The first C-linkage function, which was invoked as a native method, invoked another C-linkage function. The second C-linkage function invoked a Java method through the native method interface. This Java method invoked another Java method, which is the current method shown in the figure.

As with the other runtime memory areas, the memory they occupied by native method stacks need not be of a fixed size. It can expand and contract as needed by the running application. Implementations may allow users or programmers to specify an initial size for the method area, as well as a maximum or minimum size.