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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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Page 3 of 5

  1. invocation of instance initialization (<init>) methods
  2. invocation of private methods
  3. invocation of methods using the super keyword


Invokeinterface is used to invoke an instance method given a reference to an interface.

Method invocation of invokespecial and invokeinterface
Opcode Operand(s) Description


invokespecial
indexbyte1, indexbyte2
 pop objectref and args, invoke method at constant pool index


invokeinterface
indexbyte1, indexbyte2
 pop objectref and args, invoke method at constant pool index


The invokespecial instruction

Invokespecial differs from invokevirtual primarily in that invokespecial selects a method based on the type of the reference rather than the class of the object. In other words, it does static binding instead of dynamic binding. In each of the three situations where invokespecial is used, dynamic binding wouldn't yield the desired result.

invokespecial and <init>

The compiler places code for constructors and instance variable initializers into <init> methods, or instance initialization methods. A class gets one <init> method in the class file for each constructor in the source. If you don't explicitly declare a constructor in the source, the compiler will generate a default no-arg constructor for you. This default constructor also ends up as an <init> method in the class file. So just as every class will have at least one constructor, every class also will have at least one <init> method.

The <init> methods are called only when a new instance is created. At least one <init> method will be invoked for each class along the inheritance path of the newly created object, and multiple <init> methods could be invoked for any one class along that path.

Why is invokespecial used to invoke <init> methods? Because subclass <init> methods need to be able to invoke superclass <init> methods. This is how multiple <init> methods get invoked when an object is instantiated. The virtual machine invokes an <init> method declared in the object's class. That <init> method first invokes either another <init> method in the same class, or an <init> method in its superclass. This process continues all the way up to Object.

For example, consider this code:

class Dog {
}

class CockerSpaniel extends Dog {

    public static void main(String args[]) {
        CockerSpaniel bootsie = new CockerSpaniel();
    }
}


When you invoke main(), the virtual machine will allocate space for a new CockerSpaniel object, then invoke CockerSpaniel's default no-arg <init> method to initialize that space. That method will invoke Dog's <init> method, which will invoke Object's <init> method.

Because every class has at least one <init> method, it is common for classes to have <init> methods with identical signatures. (A method's signature is its name and the number and types of its arguments.) For example, the <init> methods for all three classes in the inheritance path for CockerSpaniel have the same signature. CockerSpaniel, Dog, and Object all contain a method named <init> that takes no arguments.

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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.