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How the Java virtual machine handles method invocation and return

A look under the hood at the bytecodes used for invoking and returning from methods

By Bill Venners, JavaWorld.com, 06/01/97

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Pushing and popping the stack frame

To invoke a method, the Java virtual machine creates a new stack frame for the method. The stack frame contains space for the method's local variables, its operand stack, and any other information required by a particular virtual machine implementation. The size of the local variables and operand stack are calculated at compile-time and placed into the class file, so the virtual machine knows just how much memory will be needed by the method's stack frame. When the JVM invokes a method, it creates a stack frame of the proper size for that method.

Adding a new frame onto the Java stack when a method is invoked is called "pushing" a stack frame; removing a frame when a method returns is called "popping" a stack frame. The Java stack is made up solely of these frames.

Invoking a Java method

If the method is a Java method (not a native method), the Java virtual machine will push a new frame onto the current Java stack.

In the case of an instance method, the virtual machine pops the objectref and args from the operand stack of the calling method's stack frame. The JVM creates a new stack frame and places the objectref on the new stack frame as local variable 0, and all the args as local variable 1, 2, and so on. The objectref is the implicit this pointer that is passed to any instance method.

For a class method, the virtual machine just pops the args from the operand stack of the calling method's frame and places them onto the new stack frame as local variable 0, 1, 2, and so on.

Once the objectref and args (or just the args, for a class method) have been placed into the local variables of the new frame, the virtual machine makes the new stack frame current and sets the program counter to point to the first instruction in the new method.

The JVM specification does not require a particular implementation for the Java stack. Frames could be allocated individually from a heap, or they could be taken from contiguous memory, or both. If two frames are contiguous, however, the virtual machine can just overlap them such that the top of the operand stack of one frame forms the bottom of the local variables of the next. In this scheme, the virtual machine need not copy objectref and args from one frame to another, because the two frames overlap. The operand stack word containing objectref in the calling method's frame would be the same memory location as local variable 0 of the new frame.

Invoking a native method

If the method being invoked is native, the Java virtual machine invokes it in an implementation-dependent manner. The virtual machine does not push a new stack frame onto the Java stack for the native method. At the point at which the thread enters the native method, it leaves the Java stack behind. When the native method returns, the Java stack once again will be used.

Other forms of method invocation

Although instance methods normally are invoked with invokevirtual, two other opcodes are used to invoke this kind of method in certain situations: invokespecial and invokeinterface.

Invokespecial is used in three situations in which an instance method must be invoked based on the type of the reference, not on the class of the object. The three situations are:

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Resources
  • The book The Java Virtual Machine Specification (http://www.aw.com/cp/lindholm-yellin.html), by Tim Lindholm and Frank Yellin (ISBN 0-201-63452-X), part of The Java Series (http://www.aw.com/cp/javaseries.html), from Addison-Wesley, is the definitive Java virtual machine reference.
  • Previous Under The Hood articles:
  • The Lean, Mean Virtual Machine -- Gives an introduction to the Java virtual machine.
  • The Java Class File Lifestyle -- Gives an overview to the Java class file, the file format into which all Java programs are compiled.
  • Java's Garbage-Collected Heap -- Gives an overview of garbage collection in general and the garbage-collected heap of the Java virtual machine in particular.
  • Bytecode Basics -- Introduces the bytecodes of the Java virtual machine, and discusses primitive types, conversion operations, and stack operations in particular.
  • Floating Point Arithmetic -- Describes the Java virtual machine's floating-point support and the bytecodes that perform floating point operations.
  • Logic and Arithmetic -- Describes the Java virtual machine's support for logical and integer arithmetic, and the related bytecodes.
  • Objects and Arrays -- Describes how the Java virtual machine deals with objects and arrays, and discusses the relevant bytecodes.
  • Exceptions -- Describes how the Java virtual machine deals with exceptions, and discusses the relevant bytecodes.
  • Try-Finally -- Describes how the Java virtual machine implements try-finally clauses, and discusses the relevant bytecodes.
  • Control Flow -- Describes how the Java virtual machine implements control flow and discusses the relevant bytecodes.
  • The Architecture of Aglets-- Describes the inner workings of Aglets, IBM's autonomous Java-based software agent technology.
  • The Point of Aglets-- Analyzes the real-world utility of mobile agents such as Aglets, IBM's autonomous Java-based software agent technology.