Java并发编程---ThreadLocal和单例模型

 一.ThreadLocal

     1.概念

      线程局部变量,是一种多线程间并发访问变量的解决方案.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不具备绝对的优势,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.

       2.代码示例

package com.thread.local;

public class ConnThreadLocal {
/**
* ThreadLocal只针对一个线程有效,在一个线程内有效,未必也对另一个线程有效
*/
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) {
final ConnThreadLocal ct = new ConnThreadLocal();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
// TODO Auto-generated method stub
ct.setTh("张三");
ct.getTh();
}
},"t1");

Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
// TODO Auto-generated method stub
try {
Thread.sleep(1000);
ct.getTh();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
},"t2");

t1.start();
t2.start();
}

}

 二.单例&多线程

      2.1 概念

            单例模式,最常见的就是饥饿模式和懒汉模式,一个直接实例化对象,一个在调用方法时实例化对象.在多线程模式中,考虑到性能和线程安全问题,我们一般选择下面两种比较经典的单例模式.在性能提高的同时,又保证了线程安全.

            double  check  instance

            static    inner    class

      2.2  代码示例

package com.thread.local;

public class DubbleSingletop {

private static DubbleSingletop ds;

public static DubbleSingletop getDs(){
if(ds == null){
try {
//模拟初始化对象的准备时间
Thread.sleep(3000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
synchronized (DubbleSingletop.class) {
if(ds == null){
ds = new DubbleSingletop();
}
}
}
return ds;
}
public static void main(String[] args) {
// TODO Auto-generated method stub
Thread t1 = new Thread(new Runnable() {

@Override
public void run() {
// TODO Auto-generated method stub
System.out.println(DubbleSingletop.getDs().hashCode());
}
},"t1");

Thread t2 = new Thread(new Runnable() {

@Override
public void run() {
// TODO Auto-generated method stub
System.out.println(DubbleSingletop.getDs().hashCode());
}
},"t2");

Thread t3 = new Thread(new Runnable() {

@Override
public void run() {
// TODO Auto-generated method stub
System.out.println(DubbleSingletop.getDs().hashCode());
}
},"t3");

t1.start();
t2.start();
t3.start();
}

}

package com.thread.local;

/**
* 单例模式:跟类一起初始化,天然的就是线程安全
* @author lhy
* @time   2017.12.26
*/
public class Singletion {

private static class InnerSingletion{
private static Singletion single = new Singletion();
}

public static Singletion getInstance(){
return InnerSingletion.single;
}
}