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1 饿汉式

/** * @author ：wyaoyao * @date ： 2020-04-06 14:32 * 单例模式：饿汉式 * 线程安全 * 但是在不使用的时候占用内存，因为通过static是类主动加载，不能能够懒加载 */public class SingletonSimple1 {       private static final SingletonSimple1 instance = new SingletonSimple1();    // 构造私有化。不允许外部调用    private SingletonSimple1() {       }    public static SingletonSimple1 getInstance() {           return instance;    }}

• 线程安全
• 但是在不使用的时候占用内存，通过static是类主动加载，不能能够懒加载

测试

// 测试@Testpublic void test() {       SingletonSimple1 instance1 = SingletonSimple1.getInstance();    SingletonSimple1 instance2 = SingletonSimple1.getInstance();    Assert.assertTrue(instance1 == instance2);}

懒汉式

/** * @author ：wyaoyao * @date ： 2020-04-06 14:41 * 饿汉式：懒加载 */public class SingletonSimple2 {       private static SingletonSimple2 instance;    private SingletonSimple2(){       }    // 存在线程安全问题    public static SingletonSimple2 getInstance(){           if(null == instance){               instance = new SingletonSimple2();        }        return instance;    }}

• 这个懒加载，在使用的时候才会创建对象，但是线程不安全

测试

// 测试，构建100线程去创建对象，测试是否线程安全@Testpublic void test2() {       for (int i = 0; i<100; i++){           new Thread(()->{               SingletonSimple2 instance = SingletonSimple2.getInstance();            System.out.println(instance);        },"T"+i).start();    }    /**     * 不安全，并不是单例的，SingletonSimple2@608f35ee，SingletonSimple2@5617e729     * study.wyy.concurrency.designpatterns.singleton.SingletonSimple2@608f35ee     * study.wyy.concurrency.designpatterns.singleton.SingletonSimple2@5617e729     * study.wyy.concurrency.designpatterns.singleton.SingletonSimple2@7df5ed4e     * study.wyy.concurrency.designpatterns.singleton.SingletonSimple2@7df5ed4e     * study.wyy.concurrency.designpatterns.singleton.SingletonSimple2@7df5ed4e     */}

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解决懒汉式的线程不安全

方案一

// 加锁    public synchronized static SingletonSimple2 getInstance(){           if(null == instance){               instance = new SingletonSimple2();        }        return instance;    }

问题

• 除了第一次是创建，以后都是读的操作，每次都会加锁，效率不高

方案二

double check

public class SingletonSimple3 {       private static SingletonSimple3 instance;    private SingletonSimple3(){       }    public static  SingletonSimple3 getInstance(){           if(null == instance){               synchronized (SingletonSimple3.class){                   if(null == instance){                       instance = new SingletonSimple3();                }            }        }        return SingletonSimple3.instance;    }}

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也存在问题：可能会引起npe

解决这个问题很简单，使用volatile关键字即可

public class SingletonSimple3 {       private static volatile SingletonSimple3 instance;    private SingletonSimple3(){       }    public static  SingletonSimple3 getInstance(){           if(null == instance){               synchronized (SingletonSimple3.class){                   if(null == instance){                       instance = new SingletonSimple3();                }            }        }        return SingletonSimple3.instance;    }}

ClassHolder

• 支持懒加载
• 线程安全
• 不会npe

public class SingletonSimple5 {       private SingletonSimple5(){       }    private static class InstanceHolder {           private final static SingletonSimple5 instance = new SingletonSimple5();    }    public static SingletonSimple5 getInstance(){           return InstanceHolder.instance;    }}

枚举方式

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