Java:使用synchronized和Lock对象获取对象锁

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

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

结果：
[align=left]Thread-0:not synchronized in f()[/align]
[align=left]Thread-0:synchronized in f()[/align]
[align=left]main:not synchronized in h()[/align]
[align=left]Thread-1:not synchronized in g()[/align]
[align=left]Thread-0:synchronized in f()[/align]
[align=left]Thread-0:synchronized in f()[/align]
[align=left]Thread-0:synchronized in f()[/align]
[align=left]Thread-0:synchronized in f()[/align]
[align=left]Thread-1:synchronized in g()[/align]
[align=left]Thread-1:synchronized in g()[/align]
[align=left]Thread-1:synchronized in g()[/align]
[align=left]Thread-1:synchronized in g()[/align]
[align=left]Thread-1:synchronized in g()[/align]
[align=left]main:synchronized in h()[/align]
[align=left]main:synchronized in h()[/align]
[align=left]main:synchronized in h()[/align]
[align=left]main:synchronized in h()[/align]
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

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

结果：
[align=left]Thread-0:not synchronized in f()[/align]
[align=left]Thread-0:synchronized in f()[/align]
[align=left]main:not synchronized in h()[/align]
[align=left]main:synchronized in h()[/align]
[align=left]Thread-1:not synchronized in g()[/align]
[align=left]Thread-1:synchronized in g()[/align]
[align=left]Thread-0:synchronized in f()[/align]
[align=left]main:synchronized in h()[/align]
[align=left]Thread-1:synchronized in g()[/align]
[align=left]Thread-0:synchronized in f()[/align]
[align=left]main:synchronized in h()[/align]
[align=left]Thread-1:synchronized in g()[/align]
[align=left]Thread-0:synchronized in f()[/align]
[align=left]main:synchronized in h()[/align]
[align=left]Thread-1:synchronized in g()[/align]
[align=left]Thread-0:synchronized in f()[/align]
[align=left]main:synchronized in h()[/align]
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

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

结果：
[align=left]Thread-0:not synchronized in f()[/align]
[align=left]Thread-0:synchronized in f()[/align]
[align=left]main:not synchronized in h()[/align]
[align=left]Thread-1:not synchronized in g()[/align]
[align=left]Thread-0:synchronized in f()[/align]
[align=left]Thread-0:synchronized in f()[/align]
[align=left]Thread-0:synchronized in f()[/align]
[align=left]Thread-0:synchronized in f()[/align]
[align=left]main:synchronized in h()[/align]
[align=left]main:synchronized in h()[/align]
[align=left]main:synchronized in h()[/align]
[align=left]main:synchronized in h()[/align]
[align=left]main:synchronized in h()[/align]
[align=left]Thread-1:synchronized in g()[/align]
[align=left]Thread-1:synchronized in g()[/align]
[align=left]Thread-1:synchronized in g()[/align]
[align=left]Thread-1:synchronized in g()[/align]
Thread-1:synchronized in g()
Resource4.java

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

结果：
[align=left]Thread-0:not synchronized in f()[/align]
[align=left]Thread-0:synchronized in f()[/align]
[align=left]main:not synchronized in h()[/align]
[align=left]main:synchronized in h()[/align]
[align=left]Thread-1:not synchronized in g()[/align]
[align=left]Thread-1:synchronized in g()[/align]
[align=left]Thread-0:synchronized in f()[/align]
[align=left]main:synchronized in h()[/align]
[align=left]Thread-1:synchronized in g()[/align]
[align=left]Thread-0:synchronized in f()[/align]
[align=left]main:synchronized in h()[/align]
[align=left]Thread-1:synchronized in g()[/align]
[align=left]Thread-0:synchronized in f()[/align]
[align=left]main:synchronized in h()[/align]
[align=left]Thread-1:synchronized in g()[/align]
[align=left]Thread-0:synchronized in f()[/align]
[align=left]main:synchronized in h()[/align]
Thread-1:synchronized in g()