Java:多线程,线程同步,同步锁(Lock)的使用(ReentrantLock、ReentrantReadWriteLock)
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