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2.9. Special Methods

At the level of the Java Virtual Machine, every constructor written in the Java programming language (JLS §8.8) appears as an instance initialization method that has the special name <init>. This name is supplied by a compiler. Because the name <init> is not a valid identifier, it cannot be used directly in a program written in the Java programming language. Instance initialization methods may be invoked only within the Java Virtual Machine by the invokespecial instruction (§invokespecial), and they may be invoked only on uninitialized class instances. An instance initialization method takes on the access permissions (JLS §6.6) of the constructor from which it was derived.

A class or interface has at most one class or interface initialization method and is initialized (§5.5) by invoking that method. The initialization method of a class or interface has the special name <clinit>, takes no arguments, and is void (§4.3.3).

  • Other methods named <clinit> in a class file are of no consequence. They are not class or interface initialization methods. They cannot be invoked by any Java Virtual Machine instruction and are never invoked by the Java Virtual Machine itself.

In a class file whose version number is 51.0 or above, the method must additionally have its ACC_STATIC flag (§4.6) set in order to be the class or interface initialization method.

  • This requirement is new in Java SE 7. In a class file whose version number is 50.0 or below, a method named <clinit> that is void and takes no arguments is considered the class or interface initialization method regardless of the setting of its ACC_STATIC flag.

The name <clinit> is supplied by a compiler. Because the name <clinit> is not a valid identifier, it cannot be used directly in a program written in the Java programming language. Class and interface initialization methods are invoked implicitly by the Java Virtual Machine; they are never invoked directly from any Java Virtual Machine instruction, but are invoked only indirectly as part of the class initialization process.

A method is signature polymorphic if and only if all of the following conditions hold:

  • It is declared in the java.lang.invoke.MethodHandle class.
  • It has a single formal parameter of type Object[].
  • It has a return type of Object.
  • It has the ACC_VARARGS and ACC_NATIVE flags set.
  • In Java SE 7, the only signature polymorphic methods are the invoke and invokeExact methods of the class java.lang.invoke.MethodHandle.

The Java Virtual Machine gives special treatment to signature polymorphic methods in the invokevirtual instruction (§invokevirtual), in order to effect invocation of a method handle. A method handle is a typed, directly executable reference to an underlying method, constructor, field, or similar low-level operation (§5.4.3.5), with optional transformations of arguments or return values. These transformations are quite general, and include such patterns as conversion, insertion, deletion, and substitution. See the java.lang.invoke package in the Java SE platform API for more information.

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