Java 并发（JUC 包-01）

>JUC ？就是Java API 中这三个包的简称：

>atomic 包

CAS：compare and swap ？要实现原子性操作

这是一种原子性操作，这是一种原子性赋值，

比如原来的 ++i ，现在对 i 进行原子性加一，从Java5之后就有了这个原子性操作，主要借助unsafe类（源码无法查看），Java为我们封装了一些类：

在atomic 包中

之前的例子改一下：

public class TestAtomic {V v = new V();public static void main(String[] args) {TestAtomic ta = new TestAtomic();ta.go();}private void go() {for (int i = 0; i < 3; i++) {new Thread(new Runnable() {public void run() {for (int j = 0; j < 10000; j++) {v.add();}}}).start();}while (Thread.activeCount() > 1) {}System.out.println(v.ai.get());}class V {//public int i = 0;public AtomicInteger ai=new AtomicInteger();public void add() {//原本在这个地方加个锁，将整个方法锁住了，悲观锁（重量级锁）//如果i++ 变成原子性的就没必要在外部加锁了//i++;ai.getAndIncrement();}}
}

>locks 包

1.重入锁基本使用：

public class TestLock {public static void main(String[] args) {new TestLock().go();}public void go() {V v = new V();new Thread(new Runnable() {public void run() {while (true) {v.printString("AAAAAAAAAAAAAA");}}}).start();new Thread(new Runnable() {public void run() {while (true) {v.printString("BBBBBBBBBBBBBB");}}}).start();}class V {// 使用Lock 代替 synchronizedReentrantLock lock = new ReentrantLock();public void printString(String str) {try {lock.lock();for (int i = 0; i < str.length(); i++) {System.out.print(str.charAt(i));}System.out.println();} catch (Exception e) {} finally {// 为了确保锁可以被释放掉lock.unlock();}}}
}

2.条件锁：使用条件锁实现线程轮询

public class TestConditionLock {V v = new V();public static void main(String[] args) {TestConditionLock t = new TestConditionLock();t.go();}private void go() {new Thread(new Runnable() {@Overridepublic void run() {while (true) {v.f1();}}}).start();new Thread(new Runnable() {@Overridepublic void run() {while (true) {v.f2();}}}).start();new Thread(new Runnable() {@Overridepublic void run() {while (true) {v.f3();}}}).start();}class V {Lock lock = new ReentrantLock();Condition condition1 = lock.newCondition();Condition condition2 = lock.newCondition();Condition condition3 = lock.newCondition();int token = 1;public void f1() {lock.lock();try {while (token != 1) {try {condition1.await();} catch (InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();}}token = 2;System.out.println(Thread.currentThread().getName() + "--f1");condition2.signal();} catch (Exception e) {e.printStackTrace();} finally {lock.unlock();}}public void f2() {lock.lock();try {while (token != 2) {try {condition2.await();} catch (InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();}}token = 3;System.out.println(Thread.currentThread().getName() + "--f2");condition3.signal();} catch (Exception e) {e.printStackTrace();} finally {lock.unlock();}}public void f3() {lock.lock();try {while (token != 3) {try {condition3.await();} catch (InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();}}token = 1;System.out.println(Thread.currentThread().getName() + "--f3");condition1.signal();} catch (Exception e) {e.printStackTrace();} finally {lock.unlock();}}}
}

3.读写锁：

读写锁分为读锁和写锁。多个读锁之间不互斥、读锁和写锁之间互斥、写锁和写锁之间互斥。这一功能很重要，对并发读取提高了性能。

读写锁使用到了JUC中的ReentrantReadWriteLock类：

/*** 测试读写锁： 1.有个地方存东西 2.提供读和写两个方法，对这两个地方进行操作 3.开启多个线程，有读，有写 4.证明读的时候可以读，写的时候不能其他操作*/
public class TestReentrantReadWriteLock {public static void main(String[] args) {TestReentrantReadWriteLock t = new TestReentrantReadWriteLock();t.go();}Data d = new Data();Random r = new Random();private void go() {for (int i = 0; i < 3; i++) {new Thread(new Runnable() {@Overridepublic void run() {d.write(r.nextInt(1000));}}).start();}for (int i = 0; i < 5; i++) {new Thread(new Runnable() {@Overridepublic void run() {d.read(r.nextInt(999));}}).start();}}class Data {List<Integer> list = new ArrayList<>();{for (int i = 0; i < 1000; i++) {list.add(i);}}// 创建读写锁ReadWriteLock lock = new ReentrantReadWriteLock();Lock readLock = lock.readLock();Lock writeLock = lock.writeLock();public void read(int index) {readLock.lock();try {System.out.println(Thread.currentThread().getName() + "进入读方法");try {Thread.sleep(1000);} catch (InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();}int i = list.get(index);System.out.println(Thread.currentThread().getName() + "读到数据" + i);System.out.println(Thread.currentThread().getName() + "读取完毕");} catch (Exception e) {// TODO Auto-generated catch blocke.printStackTrace();} finally {readLock.unlock();}}public void write(int data) {writeLock.lock();try {System.out.println(Thread.currentThread().getName() + "进入写方法");try {Thread.sleep(1000);} catch (InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();}list.add(data);System.out.println(Thread.currentThread().getName() + "写入完毕");} catch (Exception e) {// TODO Auto-generated catch blocke.printStackTrace();} finally {writeLock.unlock();}}}
}

显示：

Thread-0进入写方法
Thread-0写入完毕
Thread-1进入写方法
Thread-1写入完毕
Thread-2进入写方法
Thread-2写入完毕
Thread-3进入读方法
Thread-4进入读方法
Thread-5进入读方法
Thread-6进入读方法
Thread-7进入读方法
Thread-4读到数据403
Thread-5读到数据276
Thread-5读取完毕
Thread-7读到数据98
Thread-7读取完毕
Thread-6读到数据628
Thread-6读取完毕
Thread-3读到数据258
Thread-3读取完毕
Thread-4读取完毕

数据库中有表级别锁，行级锁，如何实现？思考。

单例模式中的同步代码块如何取代：

先看API实例：

class CachedData {Object data;volatile boolean cacheValid;final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();void processCachedData() {rwl.readLock().lock();if (!cacheValid) {// Must release read lock before acquiring write lockrwl.readLock().unlock();rwl.writeLock().lock();try {// Recheck state because another thread might have// acquired write lock and changed state before we did.if (!cacheValid) {data = ...cacheValid = true;}// Downgrade by acquiring read lock before releasing write lockrwl.readLock().lock();} finally {rwl.writeLock().unlock(); // Unlock write, still hold read}}try {use(data);} finally {rwl.readLock().unlock();}}}

改造一下单例：

public class Book {private static Book instance = null;private static ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();public static Book getInstance() {try {rwl.readLock().lock();if (instance == null) {rwl.readLock().unlock();rwl.writeLock().lock();if (instance == null) {instance = new Book();}rwl.readLock().lock();rwl.writeLock().unlock();}} catch (Exception e) {// TODO Auto-generated catch blocke.printStackTrace();} finally {rwl.readLock().unlock();}return instance;}
}

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Java 并发（JUC 包-01）

>JUC ？就是Java API 中这三个包的简称：

>atomic 包

>locks 包

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Java 并发（JUC 包-01）

>JUC ？ 就是Java API 中这三个包的简称：

>atomic 包

>locks 包

Java 并发（JUC 包-01）相关推荐

最新文章

热门文章

>JUC ？就是Java API 中这三个包的简称：