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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

This month's Under The Hood focuses on method invocation and return inside the Java virtual machine (JVM). It describes the four ways Java (and native) methods can be invoked, gives a code sample that illustrates the four ways, and covers the relevant bytecodes.

Method invocation

The Java programming language provides two basic kinds of methods: instance methods and class (or static) methods. The difference between these two kinds of methods are:

1. Instance methods require an instance before they can be invoked, whereas class methods do not.
2. Instance methods use dynamic (late) binding, whereas class methods use static (early) binding.

When the Java virtual machine invokes a class method, it selects the method to invoke based on the type of the object reference, which is always known at compile-time. On the other hand, when the virtual machine invokes an instance method, it selects the method to invoke based on the actual class of the object, which may only be known at run time.

The JVM uses two different instructions, shown in the following table, to invoke these two different kinds of methods: invokevirtual for instance methods, and invokestatic for class methods.

Method invocation of invokevirtual and invokestatic

Opcode	Operand(s)	Description

invokevirtual

indexbyte1, indexbyte2

pop objectref and args, invoke method at constant pool index

invokestatic

indexbyte1, indexbyte2

pop args, invoke static method at constant pool index

Dynamic linking

Because Java programs are dynamically linked, references to methods initially are symbolic. All invoke instructions, such as invokevirtual and invokestatic, refer to a constant pool entry that initially contains a symbolic reference. (See my earlier column, "The Java class file lifestyle," for a description of constant pool.) The symbolic reference is a bundle of information that uniquely identifies a method, including the class name, method name, and method descriptor. (A method descriptor is the method's return type and the number and types of its arguments.) The first time the Java virtual machine encounters a particular invoke instruction, the symbolic reference must be resolved.

To resolve a symbolic reference, the JVM locates the method being referred to symbolically and replaces the symbolic reference with a direct reference. A direct reference, such as a pointer or offset, allows the virtual machine to invoke the method more quickly if the reference is ever used again in the future.

For example, upon encountering an invokevirtual instruction, the Java virtual machine forms an index into the constant pool of the current class from the indexbyte1 and indexbyte2 operands that follow the invokevirtual opcode. The constant pool entry contains a symbolic reference to the method to invoke. The process of resolving symbolic references in the constant pool is how the JVM performs dynamic linking.

Verification

During resolution, the JVM also performs several verification checks. These checks ensure that Java language rules are followed and that the invoke instruction is safe to execute. For example, the virtual machine first makes sure the symbolically referenced method exists. If it exists, the virtual machine checks to make sure the current class can legally access the method. For example, if the method is private, it must be a member of the current class. If any of these checks fail, the Java virtual machine throws an exception.

Objectref and args

Once the method has been resolved, the Java virtual machine is ready to invoke it. If the method is an instance method, it must be invoked on an object. For every instance method invocation, the virtual machine expects a reference to the object (objectref) to be on the stack. In addition to objectref, the virtual machine expects the arguments (args) required by the method, if any, to be on the stack. If the method is a class method, only the args are on the stack. Class methods don't require an objectref because they aren't invoked on an object.

The objectref and args (or just args, in the case of a class method) must be pushed onto the calling method's operand stack by the bytecode instructions that precede the invoke instruction.