Java:施用synchronized和Lock对象获取对象锁

Java:使用synchronized和Lock对象获取对象锁
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在并发环境下，解决共享资源冲突问题时，可以考虑使用锁机制。
1.对象的锁
所有对象都自动含有单一的锁。
JVM负责跟踪对象被加锁的次数。如果一个对象被解锁，其计数变为0。在任务（线程）第一次给对象加锁的时候，计数变为1。每当这个相同的任务（线程）在此对象上获得锁时，计数会递增。
只有首先获得锁的任务（线程）才能继续获取该对象上的多个锁。
每当任务离开一个synchronized方法，计数递减，当计数为0的时候，锁被完全释放，此时别的任务就可以使用此资源。
2.synchronized同步块
2.1同步到单一对象锁
当使用同步块时，如果方法下的同步块都同步到一个对象上的锁，则所有的任务（线程）只能互斥的进入这些同步块。
Resource1.java演示了三个线程（包括main线程）试图进入某个类的三个不同的方法的同步块中，虽然这些同步块处在不同的方法中，但由于是同步到同一个对象（当前对象 synchronized (this)），所以对它们的方法依然是互斥的。
Resource1.java

package com.zj.lock;import java.util.concurrent.TimeUnit;?public class Resource1 {??? public void f() {?????? // other operations should not be locked...?????? System.out.println(Thread.currentThread().getName()????????????? + ":not synchronized in f()");?????? synchronized (this) {?????????? for (int i = 0; i &lt; 5; i++) {????????????? System.out.println(Thread.currentThread().getName()???????????????????? + ":synchronized in f()");????????????? try {????????????????? TimeUnit.SECONDS.sleep(3);????????????? } catch (InterruptedException e) {????????????????? e.printStackTrace();????????????? }?????????? }?????? }??? }???? public void g() {?????? // other operations should not be locked...?????? System.out.println(Thread.currentThread().getName()????????????? + ":not synchronized in g()");?????? synchronized (this) {?????????? for (int i = 0; i &lt; 5; i++) {????????????? System.out.println(Thread.currentThread().getName()???????????????????? + ":synchronized in g()");????????????? try {????????????????? TimeUnit.SECONDS.sleep(3);????????????? } catch (InterruptedException e) {????????????????? e.printStackTrace();????????????? }?????????? }?????? }??? }???? public void h() {?????? // other operations should not be locked...?????? System.out.println(Thread.currentThread().getName()????????????? + ":not synchronized in h()");?????? synchronized (this) {?????????? for (int i = 0; i &lt; 5; i++) {????????????? System.out.println(Thread.currentThread().getName()???????????????????? + ":synchronized in h()");????????????? try {????????????????? TimeUnit.SECONDS.sleep(3);????????????? } catch (InterruptedException e) {????????????????? e.printStackTrace();????????????? }?????????? }?????? }??? }???? public static void main(String[] args) {?????? final Resource1 rs = new Resource1();??????? new Thread() {?????????? public void run() {????????????? rs.f();?????????? }?????? }.start();??????? new Thread() {?????????? public void run() {????????????? rs.g();?????????? }?????? }.start();??????? rs.h();??? }}

结果：
Thread-0:not synchronized in f()
Thread-0:synchronized in f()
main:not synchronized in h()
Thread-1:not synchronized in g()
Thread-0:synchronized in f()
Thread-0:synchronized in f()
Thread-0:synchronized in f()
Thread-0:synchronized in f()
Thread-1:synchronized in g()
Thread-1:synchronized in g()
Thread-1:synchronized in g()
Thread-1:synchronized in g()
Thread-1:synchronized in g()
main:synchronized in h()
main:synchronized in h()
main:synchronized in h()
main:synchronized in h()
main:synchronized in h()
2.2 同步到多个对象锁
Resource1.java演示了三个线程（包括main线程）试图进入某个类的三个不同的方法的同步块中，这些同步块处在不同的方法中，并且是同步到三个不同的对象（synchronized (this)，synchronized (syncObject1)，synchronized (syncObject2)），所以对它们的方法中的临界资源访问是独立的。
Resource2.java

package com.zj.lock;import java.util.concurrent.TimeUnit;?public class Resource2 {??? private Object syncObject1 = new Object();??? private Object syncObject2 = new Object();???? public void f() {?????? // other operations should not be locked...?????? System.out.println(Thread.currentThread().getName()????????????? + ":not synchronized in f()");?????? synchronized (this) {?????????? for (int i = 0; i &lt; 5; i++) {????????????? System.out.println(Thread.currentThread().getName()???????????????????? + ":synchronized in f()");????????????? try {????????????????? TimeUnit.SECONDS.sleep(3);????????????? } catch (InterruptedException e) {????????????????? e.printStackTrace();????????????? }?????????? }?????? }??? }???? public void g() {?????? // other operations should not be locked...?????? System.out.println(Thread.currentThread().getName()????????????? + ":not synchronized in g()");?????? synchronized (syncObject1) {?????????? for (int i = 0; i &lt; 5; i++) {????????????? System.out.println(Thread.currentThread().getName()???????????????????? + ":synchronized in g()");????????????? try {????????????????? TimeUnit.SECONDS.sleep(3);????????????? } catch (InterruptedException e) {????????????????? e.printStackTrace();????????????? }?????????? }?????? }??? }???? public void h() {?????? // other operations should not be locked...?????? System.out.println(Thread.currentThread().getName()????????????? + ":not synchronized in h()");?????? synchronized (syncObject2) {?????????? for (int i = 0; i &lt; 5; i++) {????????????? System.out.println(Thread.currentThread().getName()???????????????????? + ":synchronized in h()");????????????? try {????????????????? TimeUnit.SECONDS.sleep(3);????????????? } catch (InterruptedException e) {????????????????? e.printStackTrace();????????????? }?????????? }?????? }??? }???? public static void main(String[] args) {?????? final Resource2 rs = new Resource2();??????? new Thread() {?????????? public void run() {?????? ?????? rs.f();?????????? }?????? }.start();??????? new Thread() {?????????? public void run() {????????????? rs.g();?????????? }?????? }.start();??????? rs.h();??? }}

结果：
Thread-0:not synchronized in f()
Thread-0:synchronized in f()
main:not synchronized in h()
main:synchronized in h()
Thread-1:not synchronized in g()
Thread-1:synchronized in g()
Thread-0:synchronized in f()
main:synchronized in h()
Thread-1:synchronized in g()
Thread-0:synchronized in f()
main:synchronized in h()
Thread-1:synchronized in g()
Thread-0:synchronized in f()
main:synchronized in h()
Thread-1:synchronized in g()
Thread-0:synchronized in f()
main:synchronized in h()
Thread-1:synchronized in g()
3.Lock对象锁
除了使用synchronized外，还可以使用Lock对象来创建临界区。Resource3.java的演示效果同Resource1.java；Resource4.java的演示效果同Resource2.java。
Resource3.java

package com.zj.lock;import java.util.concurrent.TimeUnit;import java.util.concurrent.locks.Lock;import java.util.concurrent.locks.ReentrantLock;?public class Resource3 {??? private Lock lock = new ReentrantLock();???? public void f() {?????? // other operations should not be locked...?????? System.out.println(Thread.currentThread().getName()????????????? + ":not synchronized in f()");?????? lock.lock();?????? try {?????????? for (int i = 0; i &lt; 5; i++) {????????????? System.out.println(Thread.currentThread().getName()???????????????????? + ":synchronized in f()");????????????? try {????????????????? TimeUnit.SECONDS.sleep(3);????????????? } catch (InterruptedException e) {????????????????? e.printStackTrace();????????????? }?????????? }?????? } finally {?????????? lock.unlock();?????? }??? }???? public void g() {?????? // other operations should not be locked...?????? System.out.println(Thread.currentThread().getName()????????????? + ":not synchronized in g()");?????? lock.lock();?????? try {?????????? for (int i = 0; i &lt; 5; i++) {????????????? System.out.println(Thread.currentThread().getName()???????????????????? + ":synchronized in g()");????????????? try {????????????????? TimeUnit.SECONDS.sleep(3);????????????? } catch (InterruptedException e) {????????????????? e.printStackTrace();????????????? }?????????? }?????? } finally {?????????? lock.unlock();?????? }??? }???? public void h() {?????? // other operations should not be locked...?????? System.out.println(Thread.currentThread().getName()????????????? + ":not synchronized in h()");?????? lock.lock();?????? try {?????????? for (int i = 0; i &lt; 5; i++) {????????????? System.out.println(Thread.currentThread().getName()???????????????????? + ":synchronized in h()");????????????? try {????????????????? TimeUnit.SECONDS.sleep(3);????????????? } catch (InterruptedException e) {????????????????? e.printStackTrace();????????????? }?????????? }?????? } finally {?????????? lock.unlock();?????? }??? }???? public static void main(String[] args) {?????? final Resource3 rs = new Resource3();??????? new Thread() {?????????? public void run() {????????????? rs.f();?????????? }?????? }.start();??????? new Thread() {?????????? public void run() {????????????? rs.g();?????????? }?????? }.start();??????? rs.h();??? }}

package com.zj.lock;import java.util.concurrent.TimeUnit;import java.util.concurrent.locks.Lock;import java.util.concurrent.locks.ReentrantLock; public class Resource4 {    private Lock lock1 = new ReentrantLock();    private Lock lock2 = new ReentrantLock();    private Lock lock3 = new ReentrantLock();     public void f() {       // other operations should not be locked...       System.out.println(Thread.currentThread().getName()              + ":not synchronized in f()");       lock1.lock();       try {           for (int i = 0; i &lt; 5; i++) {              System.out.println(Thread.currentThread().getName()                     + ":synchronized in f()");              try {                  TimeUnit.SECONDS.sleep(3);              } catch (InterruptedException e) {                  e.printStackTrace();              }           }       } finally {           lock1.unlock();       }    }     public void g() {       // other operations should not be locked...       System.out.println(Thread.currentThread().getName()              + ":not synchronized in g()");       lock2.lock();       try {           for (int i = 0; i &lt; 5; i++) {              System.out.println(Thread.currentThread().getName()                     + ":synchronized in g()");              try {                  TimeUnit.SECONDS.sleep(3);              } catch (InterruptedException e) {                  e.printStackTrace();              }           }       } finally {           lock2.unlock();       }    }     public void h() {       // other operations should not be locked...       System.out.println(Thread.currentThread().getName()              + ":not synchronized in h()");       lock3.lock();       try {           for (int i = 0; i &lt; 5; i++) {              System.out.println(Thread.currentThread().getName()                     + ":synchronized in h()");              try {                  TimeUnit.SECONDS.sleep(3);              } catch (InterruptedException e) {                  e.printStackTrace();              }           }       } finally {           lock3.unlock();       }    }     public static void main(String[] args) {       final Resource4 rs = new Resource4();        new Thread() {           public void run() {              rs.f();           }       }.start();        new Thread() {           public void run() {              rs.g();           }       }.start();        rs.h();    }}

结果：
Thread-0:not synchronized in f()
Thread-0:synchronized in f()
main:not synchronized in h()
main:synchronized in h()
Thread-1:not synchronized in g()
Thread-1:synchronized in g()
Thread-0:synchronized in f()
main:synchronized in h()
Thread-1:synchronized in g()
Thread-0:synchronized in f()
main:synchronized in h()
Thread-1:synchronized in g()
Thread-0:synchronized in f()
main:synchronized in h()
Thread-1:synchronized in g()
Thread-0:synchronized in f()
main:synchronized in h()
Thread-1:synchronized in g()
本文出自 “子孑” 博客，请务必保留此出处http://zhangjunhd.blog.51cto.com/113473/70300

Java:施用synchronized和Lock对象获取

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