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Java对象锁
作者:网络转载 发布时间:[ 2016/12/28 9:55:12 ] 推荐标签:测试开发技术 Java

  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
  packagecom.zj.lock;
  importjava.util.concurrent.TimeUnit;
  publicclassResource1 {
  publicvoidf() {
  // other operations should not be locked...
  System.out.println(Thread.currentThread().getName()
  +":not synchronized in f()");
  synchronized(this) {
  for(inti = 0; i < 5; i++) {
  System.out.println(Thread.currentThread().getName()
  +":synchronized in f()");
  try{
  TimeUnit.SECONDS.sleep(3);
  }catch(InterruptedException e) {
  e.printStackTrace();
  }
  }
  }
  }
  publicvoidg() {
  // other operations should not be locked...
  System.out.println(Thread.currentThread().getName()
  +":not synchronized in g()");
  synchronized(this) {
  for(inti = 0; i < 5; i++) {
  System.out.println(Thread.currentThread().getName()
  +":synchronized in g()");
  try{
  TimeUnit.SECONDS.sleep(3);
  }catch(InterruptedException e) {
  e.printStackTrace();
  }
  }
  }
  }
  publicvoidh() {
  // other operations should not be locked...
  System.out.println(Thread.currentThread().getName()
  +":not synchronized in h()");
  synchronized(this) {
  for(inti = 0; i < 5; i++) {
  System.out.println(Thread.currentThread().getName()
  +":synchronized in h()");
  try{
  TimeUnit.SECONDS.sleep(3);
  }catch(InterruptedException e) {
  e.printStackTrace();
  }
  }
  }
  }
  publicstaticvoidmain(String[] args) {
  finalResource1 rs =newResource1();
  newThread() {
  publicvoidrun() {
  rs.f();
  }
  }.start();
  newThread() {
  publicvoidrun() {
  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
  packagecom.zj.lock;
  importjava.util.concurrent.TimeUnit;
  publicclassResource2 {
  privateObjectsyncObject1=newObject();
  privateObjectsyncObject2=newObject();
  publicvoidf() {
  // other operations should not be locked...
  System.out.println(Thread.currentThread().getName()
  +":not synchronized in f()");
  synchronized(this) {
  for(inti = 0; i < 5; i++) {
  System.out.println(Thread.currentThread().getName()
  +":synchronized in f()");
  try{
  TimeUnit.SECONDS.sleep(3);
  }catch(InterruptedException e) {
  e.printStackTrace();
  }
  }
  }
  }
  publicvoidg() {
  // other operations should not be locked...
  System.out.println(Thread.currentThread().getName()
  +":not synchronized in g()");
  synchronized(syncObject1) {
  for(inti = 0; i < 5; i++) {
  System.out.println(Thread.currentThread().getName()
  +":synchronized in g()");
  try{
  TimeUnit.SECONDS.sleep(3);
  }catch(InterruptedException e) {
  e.printStackTrace();
  }
  }
  }
  }
  publicvoidh() {
  // other operations should not be locked...
  System.out.println(Thread.currentThread().getName()
  +":not synchronized in h()");
  synchronized(syncObject2) {
  for(inti = 0; i < 5; i++) {
  System.out.println(Thread.currentThread().getName()
  +":synchronized in h()");
  try{
  TimeUnit.SECONDS.sleep(3);
  }catch(InterruptedException e) {
  e.printStackTrace();
  }
  }
  }
  }
  publicstaticvoidmain(String[] args) {
  finalResource2 rs =newResource2();
  newThread() {
  publicvoidrun() {
  rs.f();
  }
  }.start();
  newThread() {
  publicvoidrun() {
  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
  packagecom.zj.lock;
  importjava.util.concurrent.TimeUnit;
  importjava.util.concurrent.locks.Lock;
  importjava.util.concurrent.locks.ReentrantLock;
  publicclassResource3 {
  privateLocklock=newReentrantLock();
  publicvoidf() {
  // other operations should not be locked...
  System.out.println(Thread.currentThread().getName()
  +":not synchronized in f()");
  lock.lock();
  try{
  for(inti = 0; i < 5; i++) {
  System.out.println(Thread.currentThread().getName()
  +":synchronized in f()");
  try{
  TimeUnit.SECONDS.sleep(3);
  }catch(InterruptedException e) {
  e.printStackTrace();
  }
  }
  }finally{
  lock.unlock();
  }
  }
  publicvoidg() {
  // other operations should not be locked...
  System.out.println(Thread.currentThread().getName()
  +":not synchronized in g()");
  lock.lock();
  try{
  for(inti = 0; i < 5; i++) {
  System.out.println(Thread.currentThread().getName()
  +":synchronized in g()");
  try{
  TimeUnit.SECONDS.sleep(3);
  }catch(InterruptedException e) {
  e.printStackTrace();
  }
  }
  }finally{
  lock.unlock();
  }
  }

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