invokespecial is used in certain special cases to invoke a
method Specifically, invokespecial is used to invoke:
the instance initialization method, <init>
a private method of this
a method in a superclass of this
The main use of invokespecial is to invoke an object's instance
initialization method, <init>, during the construction phase for a new
object. For example, when you write in Java:
new StringBuffer()
code
like the following is generated:
new java/lang/StringBuffer ; create a new StringBuffer
dup ; make an extra reference to the new instance
; now call an instance initialization method
invokespecial java/lang/StringBuffer/<init>()V
; stack now contains an initialized StringBuffer.
invokespecial
is also used by the Java language by the 'super' keyword to access a
superclass's version of a method. For example, in the class:
class Example {
// override equals
public boolean equals(Object x) {
// call Object's version of equals
return super.equals(x);
}
}
the
'super.equals(x)' expression is compiled to:
aload_0 ; push 'this' onto the stack
aload_1 ; push the first argument (i.e. x) onto the stack
; now invoke Object's equals() method.
invokespecial java/lang/Object/equals(Ljava/lang/Object;)Z
Finally,
invokespecial is used to invoke a private method. Remember that private methods
are only visible to other methods belonging the same class as the private
method.
Before performing the method invokation, the class and the method identified by
<method-spec> are resolved. See Chapter 9 for a description of how
methods are resolved.
invokespecial first looks at the descriptor given in
<method-spec>, and determines how many argument words the method takes
(this may be zero). It pops these arguments off the operand stack. Next it pops
objectref (a reference to an object) off the operand stack.
objectref must be an instance of the class named in <method-spec>,
or one of its subclasses. The interpreter searches the list of methods defined
by the class named in <method-spec>, looking for a method called
methodname whose descriptor is descriptor. This search is not
based on the runtime type of objectref, but on the compile time type
given in <method-spec>.
Once a method has been located, invokespecial calls the method. First,
if the method is marked as synchronized, the monitor associated with
objectref is entered. Next, a new stack frame structure is established
on the call stack. Then the arguments for the method (which were popped off the
current method's operand stack) are placed in local variables of the new stack
frame structure. arg1 is stored in local variable 1, arg2 is stored in local
variable 2 and so on. objectref is stored in local variable 0 (the local
variable used for the special Java variable this). Finally, execution
continues at the first instruction in the bytecode of the new method.
Methods marked as native are handled slightly differently. For native
methods, the runtime system locates the platform-specific code for the method,
loading it and linking it into the JVM if necessary. Then the native method
code is executed with the arguments popped from the operand stack. The exact
mechanism used to invoke native methods is implementation-specific.
When the method called by invokespecial returns, any single (or
double) word return result is placed on the operand stack of the
current method. If the invoked method was marked as synchronized, the
monitor associated with objectref is exited. Execution continues at the
instruction that follows invokespecial in the bytecode.