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使用synchronized和Lock对象获取对象锁

来源:中华考试网  [2020年12月3日]  【

  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()");

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  synchronized (this) {

  for (int i = 0; i < 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 < 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 < 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 < 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 < 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 < 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 < 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 < 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 < 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();

  }

  }

  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()

  main:synchronized in h()

  main:synchronized in h()

  main:synchronized in h()

  main:synchronized in h()

  main:synchronized in h()

  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()

  Resource4.java

  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 < 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 < 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 < 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()

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