Finalizer or Finalize Methods
- http://en.wikipedia.org/wiki/Finalizer
- http://docs.oracle.com/javase/7/docs/api/java/lang/Object.html#finalize%28%29
Why?
- No guarantee that finalizers will get executed promptly
- never do anything time-critical in a finalizer
- No guarantee that they’ll get executed at all
- Severe performance penalty for using finalizers
- Object creation and deletion slowed down
- Finalizer thread was running at a lower priority than another application thread
Termination Method
- private validation method to check whether current object has been terminated,
throw IllegalStateException if it has been.
- other method runs the validation method at first
- examples
- close() method on OutputStream, InputStream, java.sql.Connection
- cancel() method on Timer
- Graphics.dispost(), Window.dispose()
Use case of Termination Method
- invoke termination method in finalizer just in case
- leave a log if current object never has not yet called termination method before finalizer calls it
FileInputStream fileInputStream = new FileInputStream();
try {
...
} catch(Exception e) {
e.printStactTrace();
} finally {
fileInputStream.close();
}
How to override Finalizer
// Manual finalizer chaining
@Override protected void finalize() throws Throwable {
try {
... // Finalize subclass state
} finally {
super.finalize();
}
}
- If a subclass implementor overrides a superclass finalizer but forgets to invoke the superclass finalizer
public class Parent {
public static void main(final String[] args) throws Exception {
doIt();
System.gc();
Thread.sleep(5000); // 5 sec sleep
}
@SuppressWarnings("unused")
private final Object guardian = new Object() {
@Override protected void finalize() {
doFinalize();
}
};
private void doFinalize() {
System.out.println("Finalize of class Parent");
}
public static void doIt() {
Child c = new Child();
System.out.println(c);
}
}
class Child extends Parent {
// Note, Child class does not call super.finalize()
@Override protected void finalize() {
System.out.println("Finalize of class Child");
}
}