What is weak reference?
To understand weak reference, first look at strong reference.
String s = new String("hello world");
This is strong reference. Only when s becomes null will the object 'new String("hello world")' become eligible for garbage collection.
Map strongMap = new HashMap();
Cat key = new Cat();
Object value = new Object();
strongMap.put(key, value);
key = null;
value = null;
The key in the map is also strong reference. Even though the key variable is set to null, the 'new Cat()' object cannot be garbage collected.
Map weakMap = new WeakHashMap();
Cat key = new Cat();
Object value = new Object();
weakMap.put(key, value);
key = null;
value = null;
Now that we put the key/value pair into a WeakHashMap, the 'new Cat()' object does become garbage collected once the key is set to null.
Here is a complete program to illustrate the difference.
First a utility class
private static class MyKey {
@Override
protected void finalize() throws Throwable {
super.finalize();
System.out.println("Gabage collected");
}
}
StrongMap
public static void main(String[] args) {
Map<MyKey, Object> strongMap = new HashMap<MyKey, Object>();
for (int i = 0; i < 1000; i++){
for (int j=0; j < 10000; j++){
MyKey key = new MyKey();
Object value = new Object();
strongMap.put(key, value);
}
}
}
Output is OutOfMemory error
WeakMap
public static void main(String[] args) {
Map<MyKey, Object> weakMap = new WeakHashMap<MyKey, Object>();
for (int i = 0; i < 1000; i++){
for (int j=0; j < 10000; j++){
MyKey key = new MyKey();
Object value = new Object();
weakMap.put(key, value);
}
}
}
Output is tons of "Gabage collected" message and you don't see an OutOfMemory error.
