深入探讨 java.lang.ref 包（转）

String tag = new String("T");

SoftReference<Bean> bean = new SoftReference<Bean>(new Bean("name", 10));
System.out.println(bean.get());// “name:10”

public static void main(String[] args) {
ReferenceQueue<String> refQueue = new ReferenceQueue<String>();
PhantomReference<String> referent = new PhantomReference<String>(
new String("T"), refQueue);
System.out.println(referent.get());// null

System.gc();
System.runFinalization();

System.out.println(refQueue.poll() == referent); //true
}

public static void main(String[] args) {
Bean bean = new Bean("name", 10);
System.gc();
System.runFinalization();

System.out.println(bean);// “name:10”
}

public static void main(String[] args) {
SoftReference<Bean> bean = new SoftReference<Bean>(new Bean("name", 10));
System.gc();
System.runFinalization();

System.out.println(bean.get());// “name:10”
}

public static void main(String[] args) {
WeakReference<Bean> bean = new WeakReference<Bean>(new Bean("name", 10));
System.gc();
System.runFinalization();

System.out.println(bean.get());// “null”
}

public static void main(String[] args) { Bean[] referent = new Bean[100000]; for (int i=0;i<referent.length;i++){ referent[i] = new Bean("mybean:" + i,100);// 抛 Exception } }

public static void main(String[] args) { Reference<Bean>[] referent = new SoftReference[100000]; for (int i=0;i<referent.length;i++){ referent[i] = new SoftReference<Bean>(new Bean("mybean:" + i,100)); } System.out.println(referent[100].get());// “null” }

public static void main(String[] args) {
Reference<Bean>[] referent = new WeakReference[100000];
for (int i=0;i<referent.length;i++){
referent[i] = new WeakReference<Bean>(new Bean("mybean:" + i,100));
}

System.out.println(referent[100].get());// “null”
}

public static void main(String[] args) {
Reference<Bean>[] referent = new PhantomReference[100000];
ReferenceQueue<Bean> queue = new ReferenceQueue<Bean>();
for (int i=0;i<referent.length;i++){
referent[i] = new PhantomReference<Bean>(new Bean("mybean:" + i,100),
queue);// throw Exception
}

System.out.println(referent[100].get());
}

static {
ThreadGroup tg = Thread.currentThread().getThreadGroup();
for (ThreadGroup tgn = tg;
tgn != null;
tg = tgn, tgn = tg.getParent());
Thread finalizer = new FinalizerThread(tg);
finalizer.setPriority(Thread.MAX_PRIORITY - 2);
finalizer.setDaemon(true);
finalizer.start();
}

private void runFinalizer() {
synchronized (this) {
if (hasBeenFinalized()) return;
remove();
}
try {
Object finalizee = this.get();
if (finalizee != null && !(finalizee instanceof java.lang.Enum)) {
invokeFinalizeMethod(finalizee);
/* 注意，这里需要清空栈中包含该变量的的 slot,
** 从而来减少因为一个保守的 GC 实现所造成的变量未被回收的假象 */
finalizee = null;
}
} catch (Throwable x) { }
super.clear();
}

public class RefTestObj {
private int id;

public int getId() {
return id;
}

public void setId(int id) {
this.id = id;
}

@Override
public int hashCode() {
return super.hashCode();
}

@Override
public String toString() {
return super.toString() + "[id=" + this.id + "]";
}

@Override
protected void finalize() {
System.out.println("Object [" + this.hashCode() + "][
id=" + this.id + "] come into finalize");
try {
super.finalize();
} catch (Throwable e) {
e.printStackTrace();
}
}
}

import java.lang.ref.PhantomReference;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.lang.ref.WeakReference;

public class RefMainThread {
public static void main(String[] args) {
// 创建三种不同的引用类型所需对象
RefTestObj softRef = new RefTestObj();
RefTestObj weakRef = new RefTestObj();
RefTestObj phanRef = new RefTestObj();

softRef.setId(1);
weakRef.setId(2);
phanRef.setId(3);

ReferenceQueue<RefTestObj> softRefQueue = new ReferenceQueue<RefTestObj>();
ReferenceQueue<RefTestObj> weakRefQueue = new ReferenceQueue<RefTestObj>();
ReferenceQueue<RefTestObj> phanRefQueue = new ReferenceQueue<RefTestObj>();

SoftReference<RefTestObj> softRefObj =
new SoftReference<RefTestObj>(softRef, softRefQueue);
WeakReference<RefTestObj> weakRefObj =
new WeakReference<RefTestObj>(weakRef, weakRefQueue);
PhantomReference<RefTestObj> phanRefObj =
new PhantomReference<RefTestObj>(phanRef, phanRefQueue);

// 打印正常情况下三种对象引用
print(softRefObj);
print(weakRefObj);
print(phanRefObj);

// 将对象清空
softRef = null;
weakRef = null;
phanRef = null;

// 打印引用队列及 get() 方法所能取到的对象自身
if (softRefObj != null) {
System.out.println("Soft Reference Object run get():" + softRefObj.get());
System.out.println("Check soft queue:" + softRefQueue.poll());
}

if (weakRefObj != null) {
System.out.println("Weak Reference Object run get():" + weakRefObj.get());
System.out.println("Check weak queue:" + weakRefQueue.poll());
}

if (phanRefObj != null) {
System.out.println("Phantom Reference Object run get():" + phanRefObj.get());
System.out.println("Check Phantom queue:" + phanRefQueue.poll());
}

// 开始执行垃圾回收
System.gc();
System.runFinalization();

// 检查队列，是否已经被加入队列，是否还能取回对象
if (softRefObj != null) {
System.out.println("Soft Reference Object run get():" + softRefObj.get());
System.out.println("Check soft queue:" + softRefQueue.poll());
}

if (weakRefObj != null) {
System.out.println("Weak Reference Object run get():" + weakRefObj.get());
System.out.println("Check weak queue:" + weakRefQueue.poll());
}

if (phanRefObj != null) {
System.out.println("Phantom Reference Object run get():" + phanRefObj.get());
System.out.println("Check Phantom queue:" + phanRefQueue.poll());
}

// 对于虚引用对象，在经过多次 GC 之后， 才会加入到队列中去
Reference<? extends RefTestObj> mynewphan = null;
int refCount = 1;
while (mynewphan == null) {
mynewphan = phanRefQueue.poll();
System.gc();
System.runFinalization();
if (mynewphan != null) {
System.out.println("Check Phantom queue:" + mynewphan);
System.out.println("Count for " + refCount + " times");
break;
}
refCount ++;
}
}

public static void print(Reference<RefTestObj> ref) {
RefTestObj obj = ref.get();
System.out.println("The Reference is " + ref.toString() + " and with object " + obj +
" which is " + (obj == null ? "null" : "not null"));
}
}

The Reference is java.lang.ref.SoftReference@c17164 and
with object RefTestObj@1fb8ee3[id=1] which is not null
The Reference is java.lang.ref.WeakReference@61de33 and
with object RefTestObj@14318bb[id=2] which is not null
The Reference is java.lang.ref.PhantomReference@ca0b6 and with object null which is null
Soft Reference Object run get():RefTestObj@1fb8ee3[id=1]
Check soft queue:null
Weak Reference Object run get():RefTestObj@14318bb[id=2]
Check weak queue:null
Phantom Reference Object run get():null
Check Phantom queue:null
Object [27744459][id=3] come into finalize
Object [21174459][id=2] come into finalize
Soft Reference Object run get():RefTestObj@1fb8ee3[id=1]
Check soft queue:null
Weak Reference Object run get():null
Check weak queue:java.lang.ref.WeakReference@61de33
Phantom Reference Object run get():null
Check Phantom queue:null
Check Phantom queue:java.lang.ref.PhantomReference@ca0b6
Count for 2 times

The Reference is java.lang.ref.SoftReference@3a2c3a2c and
with object RefTestObj@391e391e[id=1] which is not null
The Reference is java.lang.ref.WeakReference@3a303a30 and
with object RefTestObj@39203920[id=2] which is not null
The Reference is java.lang.ref.PhantomReference@3a353a35
and with object null which is null
Soft Reference Object run get():RefTestObj@391e391e[id=1]
Check soft queue:null
Weak Reference Object run get():RefTestObj@39203920[id=2]
Check weak queue:null
Phantom Reference Object run get():null
Check Phantom queue:null
Object [958544162][id=3] come into finalize
Object [958413088][id=2] come into finalize
Soft Reference Object run get():RefTestObj@391e391e[id=1]
Check soft queue:null
Weak Reference Object run get():null
Check weak queue:java.lang.ref.WeakReference@3a303a30
Phantom Reference Object run get():null
Check Phantom queue:null
Object [958282014][id=1] come into finalize
............