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Compliant Solution (prevent garbage collection)
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This compliant solution takes into account the garbage collection issue described above. A class is not garbage collected until the {{ClassLoader}} object used to load it becomes eligible for garbage collection. An easier scheme to prevent the garbage collection is to ensure that there is a direct or indirect reference from a live thread to the singleton object that needs to be preserved. This compliant solution demonstrates this method (based on \[[Patterns 02|AA. Java References#Patterns 02]\]). that needs to be preserved. This compliant solution demonstrates this method.
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{
MySingleton ms1 = MySingleton.getInstance();
ObjectPreserver.preserveObject(ms1);
// ...
}
MySingleton ms2 = (MySingleton) ObjectPreserver.getObject();
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The {{ObjectPreserver}} class (based on \[[Patterns 02|AA. Java References#Patterns 02]\]) is shown below: |
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public class ObjectPreserver implements Runnable {
private static ObjectPreserver lifeLine = new ObjectPreserver();
// Neither this class, nor HashSet will be garbage collected.
// References from HashSet to other objects will also exhibit this property
private static HashMap<Integer,Object> protectedMap = new HashMap<Integer,Object>();
private ObjectPreserver() {
new Thread(this).start(); // keeps the reference alive
}
public synchronized void run(){
try {
wait();
} catch(InterruptedException e) { /* Forward to handler */ }
}
// Objects passed to this method will be preserved until
// the unpreserveObject method is called
public static void preserveObject(Object obj) {
protectedMap.put(0, obj);
}
// Returns the same instance every time
public static Object getObject() {
return protectedMap.get(0);
}
// Unprotect the objects so that they can be garbage collected
public static void unpreserveObject() {
protectedMap.remove(0);
}
}
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