[Java] Reference Object

时间：2010-09-13 来源：web_surf

Documents
- http://www.ibm.com/developerworks/library/j-refs/
- http://www.cnblogs.com/abob/archive/2009/10/27/1590711.html
- http://download-llnw.oracle.com/javase/1.3/docs/api/java/lang/ref/package-summary.html
Package
java.lang.ref
Objective
If you want to refer to some objects without preventing VM to collect them, you can use reference object. For example, in the statement "String obj = new String("data");", GC will not collect obj unless it goes out of scope or it is assigned to null; but if you use reference object as following:
    String obj = new ("data");
    WeakReference ref = new WeakReference(obj); //Create a reference to obj, the reference doesn't prevent VM to collect obj
    obj = null; //Inform GC to collect obj at some time
    if (ref.get() == null) //You can refer to obj with ref, but first of all, you must check if obj is collected
    {
        System.out.println("obj is not collected");
    }
    else
    {
        System.out.println("obj is collected");
    }

There are 3 types of reference:
- PhantomReference
- WeakReference
- SoftReference
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