问题
I was playing with some code to make a "closure like" construct ( not working btw )
Everything looked fine but when I tried to access a final local variable in the code, the exception InstantiationException is thrown.
If I remove the access to the local variable either by removing it altogether or by making it class attribute instead, no exception happens.
The doc says: InstantiationException
Thrown when an application tries to create an instance of a class using the newInstance method in class Class, but the specified class object cannot be instantiated. The instantiation can fail for a variety of reasons including but not limited to:
- the class object represents an abstract class, an interface, an array class, a primitive type, or void
- the class has no nullary constructor
What other reason could have caused this problem?
Here's the code. comment/uncomment the class attribute / local variable to see the effect (lines:5 and 10 ).
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
class InstantiationExceptionDemo {
//static JTextField field = new JTextField();// works if uncommented
public static void main( String [] args ) {
JFrame frame = new JFrame();
JButton button = new JButton("Click");
final JTextField field = new JTextField();// fails if uncommented
button.addActionListener( new _(){{
System.out.println("click " + field.getText());
}});
frame.add( field );
frame.add( button, BorderLayout.SOUTH );
frame.pack();frame.setVisible( true );
}
}
class _ implements ActionListener {
public void actionPerformed( ActionEvent e ){
try {
this.getClass().newInstance();
} catch( InstantiationException ie ){
throw new RuntimeException( ie );
} catch( IllegalAccessException ie ){
throw new RuntimeException( ie );
}
}
}
Is this a bug in Java?
edit
Oh, I forgot, the stacktrace ( when thrown ) is:
Caused by: java.lang.InstantiationException: InstantiationExceptionDemo$1
at java.lang.Class.newInstance0(Class.java:340)
at java.lang.Class.newInstance(Class.java:308)
at _.actionPerformed(InstantiationExceptionDemo.java:25)
回答1:
Well, that makes sense.
Only your first instance of the _ class has access to the local variable. Subsequent instances can't, unless you provide them with it (via constructor arg)
Constructor[] constructor = a.getClass().getDeclaredConstructors();
for (Constructor c : constructors) {
System.out.println(c.getParameterTypes().length);
}
outputs 1. (a is the instance of your anonymous class)
That said, I don't think this is a good way to implement closures. The initializer block is called at least once, without the need of it. I assume you are just playing around, but take a look at lambdaj. Or wait for Java 7 :)
回答2:
Here's an excerpt of the javap -c InstantiationExceptionDemo$1 of the static field version:
Compiled from "InstantiationExceptionDemo.java"
class InstantiationExceptionDemo$1 extends _{
InstantiationExceptionDemo$1();
Code:
0: aload_0
1: invokespecial #8; //Method _."<init>":()V
4: getstatic #10; //Field InstantiationExceptionDemo.field:
//Ljavax/swing/JTextField;
And here's the javap -c InstantiationExceptionDemo$1 of the final local variable version:
Compiled from "InstantiationExceptionDemo.java"
class InstantiationExceptionDemo$1 extends _{
InstantiationExceptionDemo$1(javax.swing.JTextField);
Code:
0: aload_0
1: invokespecial #8; //Method _."<init>":()V
4: aload_1
So there's your cause: the final local variable version needs an extra argument, the JTextField reference, in the constructor. It has no nullary constructor.
This makes sense if you think about it. Else, how is this version of InstantiationExceptionDemo$1 going to get the field reference? The compiler hides the fact that this is given as a parameter to the synthetic constructor.
回答3:
Thank you both Bozho and Polygenlubricants for the enlightening answers.
So, the reason is ( in my own words )
When using a local final variable, the compiler creates a constructor with the fields used by the anonymous inner class and invokes it. It also "inject" field with the values.
So, what I did, was to modify my creation to load the right constructor with the correct values using reflection.
This is the resulting code:
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import java.lang.reflect.*;
class InstantiationExceptionDemo {
public static void main( String [] args ) {
JFrame frame = new JFrame();
final JButton reverse = new JButton("Reverse");
final JButton swap = new JButton("Swap");
final JTextField fieldOne = new JTextField(20);
final JTextField fieldTwo = new JTextField(20);
// reverse the string in field one
reverse.addActionListener( new _(){{
StringBuilder toReverse = new StringBuilder();
toReverse.append( fieldOne.getText() );
toReverse.reverse();
fieldOne.setText( toReverse.toString() );
//fieldOne.setText( new StringBuilder( fieldOne.getText() ).reverse().toString() );
}});
// swap the fields
swap.addActionListener( new _(){{
String temp = fieldOne.getText();
fieldOne.setText( fieldTwo.getText() );
fieldTwo.setText( temp );
}});
// scaffolding
frame.add( new JPanel(){{
add( fieldOne );
add( fieldTwo );
}} );
frame.add( new JPanel(){{
add( reverse );
add( swap );
}}, BorderLayout.SOUTH );
frame.pack();frame.setVisible( true );
}
}
abstract class _ implements ActionListener {
public _(){}
public void actionPerformed( ActionEvent e ){
invokeBlock();
}
private void invokeBlock(){
// does actually invoke the block but with a trick
// it creates another instance of this same class
// which will be immediately discarded because there are no more
// references to it.
try {
// fields declared by the compiler in the anonymous inner class
Field[] fields = this.getClass().getDeclaredFields();
Class[] types= new Class[fields.length];
Object[] values = new Object[fields.length];
int i = 0;
for( Field f : fields ){
types[i] = f.getType();
values[i] = f.get( this );
i++;
}
// this constructor was added by the compiler
Constructor constructor = getClass().getDeclaredConstructor( types );
constructor.newInstance( values );
} catch( InstantiationException ie ){
throw new RuntimeException( ie );
} catch( IllegalAccessException ie ){
throw new RuntimeException( ie );
}catch( InvocationTargetException ie ){
throw new RuntimeException( ie );
} catch(NoSuchMethodException nsme){
throw new RuntimeException( nsme );
}
}
}
Of course, as Bozho points out, this is not a good way ( not it is a way, but not a good one ) to create closures.
There are two problems with this.
1.- The initializer block is invoked when it is declared.
2.- There is no way get the parameters of the actual code ( ie actioneEvent in actionPerformed )
If we could just delay the execution of the initializer block this would make a nice ( in terms of syntax ) closure alternative.
Perhaps in Java 7 :(
来源：https://stackoverflow.com/questions/2906954/why-am-i-having-this-instantiationexception-in-java-when-accessing-final-local-v