拓胜技术专家教你如何深入理解Java四种引用类型4

Java 2Java 2 ng.ref garbage collector Peter Haggar SoftReference WeakReference PhantomReferenceJava 2 ng.ref SoftReference WeakReference PhantomReferencestrong reference objObject obj = new Object();obj objobj nullsoft reference weak reference phantom reference ”strongly reachablesoftly reachableweakly reachablephantomly reachablereferent null SoftReferenceSoftReference SoftReference JVMWeakReferenceWeakReference canonicalized mapping WeakReferencePhantomReferencePhantomReferencepre-mortem PhantomReference ReferenceQueue ReferenceQueue PhantomReference ReferenceQueue PhantomReference ReferenceQueue PhantomReferenceSoftReference referent null heapSoftReference heap finalizableheap finalize() SoftReference ReferenceQueueWeakReference referent null heapWeakReference heap finalizableheap finalize() WeakReference ReferenceQueuePhantomReference heap finalizablePhantomReference ReferenceQueue PhantomReferenceReferenceQueue1 11. WeakReference ReferenceQueue//Create a strong reference to an objectMyObject o bj=n ew M yObject();//1//Create a reference queueReferenceQueue rq =new ReferenceQueue(); //2//Create a weakReference to obj and associate our reference queueWeakReference wr = new WeakReference(obj, rq); //31. 1 //1 //2 //31 //1 MyObject //2 ReferenceQueue //3 MyObject WeakReference ReferenceQueue obj rq wr MyObject obj null MyObjectget() ReferenceQueue poll() get() PhantomReference get() nullPhantomReference poll() null 1 get() poll()wr.get(); //returns reference to MyObjectrq.poll(); //returns nullMyObject get() poll() obj 1obj = null;System.gc(); //run the collector22. obj = null;get() poll()wr.get(); //returns nullrq.poll(); //returns a reference to the WeakReference objectMyObject WeakReference MyObject WeakReference ReferenceQueue WeakReference SoftReference get() null finalizable heap WeakReference SoftReference ReferenceQueue 22.import ng.ref.*;class MyObject{protected void finalize() throws Throwable{System.out.println("In finalize method for this object: " +this);}}class ReferenceUsage{public static void main(String args[]){hold();release();}public static void hold(){System.out.println("Example of incorrectly holding a strong " +"reference");//Create an objectMyObject obj = new MyObject();System.out.println("object is " + obj);//Create a reference queueReferenceQueue rq = new ReferenceQueue();//Create a weakReference to obj and associate our reference queueWeakReference wr = new WeakReference(obj, rq);System.out.println("The weak reference is " + wr);//Check to see if it's on the ref queue yetSystem.out.println("Polling the reference queue returns " +rq.poll());System.out.println("Getting the referent from the " +"weak r eference r eturns"+w r.get());System.out.println("Calling GC");System.gc();System.out.println("Polling the reference queue returns " +rq.poll());System.out.println("Getting the referent from the " +"weak r eference r eturns"+w r.get());}public static void release(){System.out.println("");System.out.println("Example of correctly releasing a strong " +"reference");//Create an objectMyObject obj = new MyObject();System.out.println("object is " + obj);//Create a reference queueReferenceQueue rq = new ReferenceQueue();//Create a weakReference to obj and associate our reference queueWeakReference wr = new WeakReference(obj, rq);System.out.println("The weak reference is " + wr);//Check to see if it's on the ref queue yetSystem.out.println("Polling the reference queue returns " +rq.poll());System.out.println("Getting the referent from the " +"weak r eference r eturns"+w r.get());System.out.println("Set the obj reference to null and call GC");obj =null;System.gc();System.out.println("Polling the reference queue returns " +rq.poll());System.out.println("Getting the referent from the " +"weak r eference r eturns"+w r.get());}}3 WeakReference3.obj = wr.get();if (obj == null){wr = new WeakReference(recreateIt()); //1obj=w r.get();//2}//code that works with obj4 WeakReference4.obj = wr.get();if (obj == null){obj=r ecreateIt();//1wr=n ew W eakReference(obj);//2}//code that works with obj34 3 if obj 3 //1 //2 recreateIt() WeakReference //2 wr.get() null4 //1 //2 //2 obj WeakReference if obj obj null5 “temp.fil”5.import java.io.*;import ng.ref.*;class ReferenceIdiom{public static void main(String args[]) throws FileNotFoundException {broken();correct();}public static FileReader recreateIt() throws FileNotFoundException {return new FileReader("temp.fil");}public static void broken() throws FileNotFoundException{System.out.println("Executing method broken");FileReader obj = recreateIt();WeakReference wr = new WeakReference(obj);System.out.println("wr refers to object " + wr.get());System.out.println("Now, clear the reference and run GC");//Clear the strong reference, then run GC to collect obj.obj =null;System.gc();System.out.println("wr refers to object " + wr.get());//Now see if obj was collected and recreate it if it was.obj =(FileReader)wr.get();if (obj == null){System.out.println("Now, recreate the object and wrap itin a W eakReference");wr =new WeakReference(recreateIt());System.gc(); //FileReader object is NOT pinned...there is no//strong r eference t o i t. Therefore,t he n ext//line c an r eturn n ull.obj =(FileReader)wr.get();}System.out.println("wr refers to object " + wr.get());}public static void correct() throws FileNotFoundException{System.out.println("");System.out.println("Executing method correct");FileReader obj = recreateIt();WeakReference wr = new WeakReference(obj);System.out.println("wr refers to object " + wr.get());System.out.println("Now, clear the reference and run GC");//Clear the strong reference, then run GC to collect objobj =null;System.gc();System.out.println("wr refers to object " + wr.get());//Now see if obj was collected and recreate it if it was.obj =(FileReader)wr.get();if (obj == null){System.out.println("Now, recreate the object and wrap itin a W eakReference");obj =recreateIt();System.gc(); //FileReader is pinned, this will not affect//anything.wr =new WeakReference(obj);}System.out.println("wr refers to object " + wr.get());}}。