Java:使用synchronized和Lock对象获取对象锁

2023年2月27日 291次阅读来源: brave-sailor

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 < 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(); } }

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(); } }

    原文作者：brave-sailor
    原文地址: https://www.cnblogs.com/Free-Thinker/p/4940567.html
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