java 多线程 ThreadLocal/ 单例

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ThreadLocal概念：线程局部变量，是一种多线程间并发访问的解决方案，与其synchronized等加锁的方式不同，ThreadLocal完全不提供锁，而使用以空间换时间的手段，为每个线程提供变量的独立副本，以保障线程安全。

从性能上说,ThreadLocal不具有绝对的优势，在并发不是很高的时候，加锁的性能会更好，但作为一套与锁完全无关的线程安全解决方案，在高并发量或者竞争激烈的场景，使用ThreadLocal可以在一定程度上减少锁竞争。

public class ConnThreadLocal {public static ThreadLocal<String> th = new ThreadLocal<String>();public void setTh(String value){th.set(value);}public void getTh(){System.out.println(Thread.currentThread().getName() + ":" + this.th.get());}public static void main(String[] args) throws InterruptedException {final ConnThreadLocal ct = new ConnThreadLocal();Thread t1 = new Thread(new Runnable() {@Overridepublic void run() {ct.setTh("张三");ct.getTh();}}, "t1");Thread t2 = new Thread(new Runnable() {@Overridepublic void run() {try {Thread.sleep(1000);ct.setTh("李四");ct.getTh();} catch (InterruptedException e) {e.printStackTrace();}}}, "t2");t1.start();t2.start();}}

输出结果：

t1:张三
t2:李四

public class InnerSingleton {private static class Singletion {private static Singletion single = new Singletion();}public static Singletion getInstance(){return Singletion.single;}}

public class DubbleSingleton {private static DubbleSingleton ds;public static DubbleSingleton getDs(){if(ds == null){try {//模拟初始化对象的准备时间...Thread.sleep(3000);} catch (InterruptedException e) {e.printStackTrace();}synchronized (DubbleSingleton.class) {if(ds == null){ds = new DubbleSingleton();}}}re

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