java 3种单例模式

1.饿汉式

package singleInstance;
/**饿汉式，类加载时候创建实例，浪费资源，线程是安全的*/
public class SingleDome01 {
    //私有构造方法，防止直接new对象
    private SingleDome01() {
    }
    //类加载的时候，创建对象
    private static SingleDome01 instance = new SingleDome01();
    //提供getter，方便外界访问私有对象
    public static SingleDome01 getInstance() {
        return instance;
    }
}

2.懒汉式

package singleInstance;
/**懒汉式，被调用时候才会实例化，节约资源，高并发线程时不安全*/
public class SingleDome02 {
    //私有构造方法，防止直接new对象
    private SingleDome02(){
    }
    private static SingleDome02 instance;
    //提供getter,以便外界调用私有对象，返回SingleDome02对象
    public static SingleDome02 getInstance() {
        if(instance==null){
            instance = new SingleDome02();
        }
        return instance;
    }
}

 3.饿汉式与懒汉式改良

package singleInstance;
/**采用内部类实例化对象，在调用时候才会加载，节约资源，还能保证线程是安全的*/
public class SingleDome03 {
    //私有构造方法，防止直接new对象
    private SingleDome03() {
    }
    //静态内部类只有在调用时候才会加载
    private static class InnerSingleDome03{
        private static SingleDome03 instance = new SingleDome03();
    }
    public static SingleDome03 getInstance(){
        return InnerSingleDome03.instance;
    }
}

 4.登记式

package singleInstance;

import java.util.HashMap;
import java.util.Map;

/**采用spring的用法，利用反射创建对象，讲实例注册到map里面，下次用的时候直接取*/
public class SingleDome04 {
    //私有构造方法，防止外部直接new对象
    private SingleDome04() {
    }
    private static Map<String,SingleDome04> map = new HashMap<String,SingleDome04>();
    //静态代码块,类加载时候实例化对象和map，并将对象放入map里
    static{
        SingleDome04 instance = new SingleDome04();
        map.put(instance.getClass().getName(), instance);
    }
    public static SingleDome04 getInstance(String name){
        if(name==null){
            name="singleInstance.SingleDome04";
        }
        if(!map.containsKey(name)){
            try {
                map.put(name, (SingleDome04)Class.forName(name).newInstance());
            } catch (InstantiationException e) {
                e.printStackTrace();
            } catch (IllegalAccessException e) {
                e.printStackTrace();
            } catch (ClassNotFoundException e) {
                e.printStackTrace();
            }
        }
        return map.get(name);
    }
}

最后测试代码：

package singleInstance;

public class TestSingle {
    public static void main(String[] args) {
        SingleDome01 s1 = SingleDome01.getInstance();
        SingleDome01 s2 = SingleDome01.getInstance();
        if(s1==s2){
            System.out.println("s1=s2");
        }else{
            System.out.println("s1!=s2");
        }
        SingleDome02 s3 = SingleDome02.getInstance();
        SingleDome02 s4 = SingleDome02.getInstance();
        if(s3==s4){
            System.out.println("s3=s4");
        }else{
            System.out.println("s3!=s4");
        }
        SingleDome03 s5 = SingleDome03.getInstance();
        SingleDome03 s6 = SingleDome03.getInstance();
        if(s5==s6){
            System.out.println("s5=s6");
        }else{
            System.out.println("s5!=s6");
        }
        SingleDome04 s7 = SingleDome04.getInstance("singleInstance.SingleDome04");
        SingleDome04 s8 = SingleDome04.getInstance("singleInstance.SingleDome04");
        if(s7==s8){
            System.out.println("s7=s8");
        }else{
            System.out.println("s7!=s8");
        }
    }
}