设计模式之单例模式

饱汉模式

案例一 
    创建singleton_0包

package singleton_0;

public class MyObject {
    //直接new出对象
    private static MyObject myObject = new MyObject();

    private MyObject(){

    }

    public static MyObject getInstance(){
        return myObject;
    }
}

package singleton_0;

public class MyThread extends Thread{
    @Override
    public void run(){
    System.out.println(MyObject.getInstance().hashCode());
 }
}

package singleton_0;

public class Run {
    public static void main(String[] args) {
        //启动五个线程运行
        MyThread[] thread = new MyThread[5];
        for (int i = 0; i < 5; i++) {
            thread[i] = new MyThread();
            thread[i].start();
        }
    }
}

打印出来的五个MyObject实例的哈希码都是相同的。

懒汉模式

案例二 
    复制singletion_0到singletion_1,修改MyObject.java

package singleton_1;
//懒汉模式单例
public class MyObject {
private static MyObject myObject ;

    private MyObject(){

    }
    //非同步线程不安全
    public static MyObject getInstance(){
        if(myObject != null){

        }else{
            myObject = new MyObject();
        }
        return myObject;
    }
}

显然产生了多个对象，下面做出改进，复制singleton_1包，对MyObject做如下改进

案例三 复制singletion_1到singletion_1_1，修改MyObject.java

package singleton_1_1;

public class MyObject {
private static MyObject myObject ;

    private MyObject(){

    }
    //多线程之下容易出现问题，**加上synchronized关键字**，整个方法都上锁了
    public synchronized  static MyObject getInstance(){
        if(myObject != null){
        }else{
            myObject = new MyObject();
        }
        return myObject;
    }
}

使用DCL双检查机制保证线程安全

package singleton_5;

public class MyObject {
private static MyObject myObject ;

private MyObject(){

    }
    //DCL双检查锁机制
    public static MyObject getInstance(){
        try {
            synchronized(MyObject.class){
            if(myObject != null){
            }else{
                //模拟创建对象之前做一些准备性的工作
                Thread.sleep(3000);
                //再次做同步
                synchronized(MyObject.class){
                    if(myObject == null)
                        myObject = new MyObject();
                }
            }
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return myObject;
    }
    }

使用内部类实现单例模式

package singleton_7_1;

public class MyObject {
    private static class MyObjectHandler{
        private static MyObject myObject = new MyObject();

    }

    private MyObject(){

    }
    public static MyObject getInstance(){               
        return MyObjectHandler.myObject;
    }
}