关于线程的同步，可以使用synchronized关键字，或者是使用JDK 5中提供的java.util.concurrent.lock包中的Lock对象。本文探讨Lock对象。
synchronized与java.util.concurrent.locks.Lock 的相同点：Lock能完成synchronized所实现的所有功能；主要不同点：Lock有比synchronized更精确的线程语义和更好的性能。synchronized会自动释放锁，而Lock一定要求程序员手工释放， 并且必须在finally从句中释放。
一：先来一段简单的代码
这段代码以前曾经用synchronized关键字实现同步（Java：多线程，线程同步，synchronized关键字的用法（同步代码块、非静态同步方法、静态同步方法）)，现在用Lock对象实现：
package com.clzhang.sample.thread; import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock; public class LockThread1 implements Runnable {
private Integer key = 0; // 锁对象
private Lock lock = new ReentrantLock(); @Override
public void run() {
// 需要结果是key实现自增长，如果没有同步块，则可能会出现重复key值的现象
lock.lock();
try {
key++; System.out.println(Thread.currentThread().getName() + ":" + key);
try {
Thread.sleep(10);
} catch (InterruptedException e) {
}
}finally{
// 上述代码实现功能与使用sychronized同步代码块一样。
// sychronized同步代码块或同步方法在代码执行完之后锁自动释放；而用Lock则需要手工释放锁。
// 为了保证锁最终被释放，释放锁代码放在finally块内。
lock.unlock();
}
} public static void main(String[] args) {
LockThread1 lt = new LockThread1(); for(int i=0; i<100; i++) {
new Thread(lt, "Thread" + i).start();
}
}
}
部分输出：
Thread86:95
Thread88:96
Thread90:97
Thread98:98
Thread92:99
Thread96:100
二：再来一段稍复杂的代码
package com.clzhang.sample.thread; import java.util.*; import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock; public class LockThread2 implements Runnable {
// 内部类
class Student {
private int age = 0;
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
} // 全局变量定义
private int count = 0;
private Student student = new Student(); // 锁对象
private Lock lock1 = new ReentrantLock(false);
private Lock lock2 = new ReentrantLock(false); @Override
public void run() {
String currentThreadName = Thread.currentThread().getName();
System.out.println(currentThreadName + " is running!"); lock1.lock();// 使用重入锁
System.out.println(currentThreadName + " got lock1@Step1!");
try {
count++;
Thread.sleep(3000);
} catch (Exception e) {
e.printStackTrace();
} finally {
System.out.println(currentThreadName + " first:count=" + count + "\tage=" + this.student.getAge());
lock1.unlock();
}
lock2.lock();// 使用另外一个不同的重入锁
System.out.println(currentThreadName + " got lock2@Step2!");
try {
Random random = new Random();
int age = random.nextInt(100);
this.student.setAge(age);
Thread.sleep(3000);
} catch (Exception ex) {
ex.printStackTrace();
} finally {
System.out.println(currentThreadName + " second:count=" + count + "\tage=" + this.student.getAge());
lock2.unlock();
}
} public static void main(String[] args) {
LockThread2 lt = new LockThread2();
for (int i = 1; i <= 3; i++) {
Thread t = new Thread(lt, "Thread" + i);
t.start();
}
}
}
输出：
Thread1 is running!
Thread1 got lock1@Step1! // 线程1获取锁1
Thread3 is running!
Thread2 is running!
Thread1 first:count=1 age=0
Thread3 got lock1@Step1!
Thread1 got lock2@Step2! // 线程3、1分别获取锁1、2
Thread3 first:count=2 age=13
Thread1 second:count=2 age=13 // count值已经被线程3更改；age是自己设置的。
Thread3 got lock2@Step2!
Thread2 got lock1@Step1! // 线程3、2分别获取锁2、1
Thread2 first:count=3 age=34
Thread3 second:count=3 age=34 // count值已经被线程2更改；age是自己设置的。
Thread2 got lock2@Step2! // 线程2获取锁2
Thread2 second:count=3 age=40 // 没人改count值了；age是自己设置的。
参考：http://wenku.baidu.com/view/bba6ec24482fb4daa48d4b06.html
三：再来一段读写的代码
要求：写入和写入互斥，读取和写入互斥，读取和读取之间不互斥。
package com.clzhang.sample.thread; import java.util.*; import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock; public class LockThread3 implements Runnable {
// 数据存放
private StringBuilder sb = new StringBuilder(); // 锁对象
private ReadWriteLock rwl = new ReentrantReadWriteLock(); @Override
public void run() {
if (Thread.currentThread().getName().startsWith("Read")) {
rwl.readLock().lock();// 取到读锁
try {
System.out.println(Thread.currentThread().getName() + "正在读取...");
try {
Thread.sleep(500);
} catch (InterruptedException e) {
e.printStackTrace();
} System.out.println(Thread.currentThread().getName() + "结果:" + sb.toString());
} finally {
rwl.readLock().unlock();// 释放读锁
}
} else if (Thread.currentThread().getName().startsWith("Write")) {
rwl.writeLock().lock();// 取到写锁
try {
System.out.println(Thread.currentThread().getName() + "正在写入...");
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
String writeData = "" + new Random().nextInt(100);
sb.append(writeData + ","); System.out.println(Thread.currentThread().getName() + "结果:" + writeData);
} finally {
rwl.writeLock().unlock();// 释放写锁
}
} else {
// 啥也不做行不
}
} public static void main(String[] args) {
LockThread3 lt = new LockThread3();
for (int i=0; i<5; i++) {
Thread t = new Thread(lt, "Write" + i);
t.start();
}
for (int i=0; i<5; i++) {
Thread t = new Thread(lt, "Read" + i);
t.start();
}
}
}
输出：
Write1正在写入...
Write1结果:25
Write0正在写入...
Write0结果:5
Read0正在读取...
Read2正在读取...
Read4正在读取...
Read2结果:25,5,
Read0结果:25,5,
Read4结果:25,5,
Write2正在写入...
Write2结果:77
Write4正在写入...
Write4结果:38
Read1正在读取...
Read3正在读取...
Read1结果:25,5,77,38,
Read3结果:25,5,77,38,
Write3正在写入...
Write3结果:77