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
[code]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();
}
} [/code]
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
[code]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();
}
} [/code]
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
[code]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();
}
} [/code]
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
[code]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();
}
} [/code]
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()