版权声明：本文为博主原创文章，未经博主允许不得转载。 https://blog.csdn.net/maguochao_Mark/article/details/51423725

【JAVA】三 【设计模式】一 JAVA 单例 枚举

单例特点

• 单例类只能有一个实例。
• 单例类必须自己创建自己的唯一实例。
• 单例类必须给所有其他对象提供这一实例。

懒汉

线程不安全
伪单例 可通过Java反射得到多个对象

public class Singleton {
private static Singleton instance;

private Singleton() {
}

public static Singleton getInstance() {
if (instance == null) {
instance = new Singleton();
}
return instance;
}

public static void main(String[] args) {
try {
Class cls = Single.class;
Constructor con = cls.getDeclaredConstructor(null);
con.setAccessible(true);
Object first = con.newInstance(null);
Object second = con.newInstance(null);
System.out.println(first);
System.out.println(second);
} catch (Exception e) {
e.printStackTrace();
}

}
}

懒汉

线程安全 synchronized 实现效率低
伪单例 可通过Java反射得到多个对象

package com.cn.mark.designmode;

import java.lang.reflect.Constructor;

public class Single {
private static Single instance;

private Single() {
}

public static synchronized Single getInstance() {
if (instance == null) {
instance = new Single();
}
return instance;
}

public static void main(String[] args) {
try {
Class cls = Single.class;
Constructor con = cls.getDeclaredConstructor(null);
con.setAccessible(true);
Object first = con.newInstance(null);
Object second = con.newInstance(null);
System.out.println(first);
System.out.println(second);
} catch (Exception e) {
e.printStackTrace();
}
}
}

饿汉

线程安全 instance在类装载时就实例化
伪单例 可通过Java反射得到多个对象

package com.cn.mark.designmode;

import java.lang.reflect.Constructor;

public class Single {
private static Single instance = new Single();

private Single() {
}

public static Single getInstance() {
return instance;
}

public static void main(String[] args) {
try {
Class cls = Single.class;
Constructor con = cls.getDeclaredConstructor(null);
con.setAccessible(true);
Object first = con.newInstance(null);
Object second = con.newInstance(null);
System.out.println(first);
System.out.println(second);
} catch (Exception e) {
e.printStackTrace();
}
}
}

饿汉

线程安全 static 代码块 实例化instance
伪单例 可通过Java反射得到多个对象

package com.cn.mark.designmode;

import java.lang.reflect.Constructor;

public class Single {
private static Single instance = null;
static {
instance = new Single();
}

private Single() {
}

public static Single getInstance() {
return instance;
}

public static void main(String[] args) {
try {
Class cls = Single.class;
Constructor con = cls.getDeclaredConstructor(null);
con.setAccessible(true);
Object first = con.newInstance(null);
Object second = con.newInstance(null);
System.out.println(first);
System.out.println(second);
} catch (Exception e) {
e.printStackTrace();
}
}
}

静态内部类

线程安全 SingleHolder 类加载是实例化 instance
伪单例 可通过Java反射得到多个对象

Single类被装载了，instance不一定被初始化。因为SingleHolder类没有被主动使用，只有显示通过调用getInstance方法时，才会显示装载SingleHolder类，从而实例化instance。

package com.cn.mark.designmode;

import java.lang.reflect.Constructor;

public class Single {
private static class SingleHolder {
private static final Single INSTANCE = new Single();
}

private Single() {
}

public static final Single getInstance() {
return SingleHolder.INSTANCE;
}

public static void main(String[] args) {
try {
Class cls = Single.class;
Constructor con = cls.getDeclaredConstructor(null);
con.setAccessible(true);
Object first = con.newInstance(null);
Object second = con.newInstance(null);
System.out.println(first);
System.out.println(second);
} catch (Exception e) {
e.printStackTrace();
}
}
}

DCL 双重校验锁

线程安全
伪单例 可通过Java反射得到多个对象

package com.cn.mark.designmode;

import java.lang.reflect.Constructor;

/**
* Singleton pattern example with Double checked Locking
*/
public class DoubleCheckedLockingSingleton {
private volatile DoubleCheckedLockingSingleton INSTANCE;

private DoubleCheckedLockingSingleton() {
}

public DoubleCheckedLockingSingleton getInstance() {
if (INSTANCE == null) {
synchronized (DoubleCheckedLockingSingleton.class) {
// double checking Singleton instance
if (INSTANCE == null) {
INSTANCE = new DoubleCheckedLockingSingleton();
}
}
}
return INSTANCE;
}

public static void main(String[] args) {
try {
Class cls = DoubleCheckedLockingSingleton.class;
Constructor con = cls.getDeclaredConstructor(null);
con.setAccessible(true);
Object first = con.newInstance(null);
Object second = con.newInstance(null);
System.out.println(first);
System.out.println(second);
} catch (Exception e) {
e.printStackTrace();
}
}
}

枚举

线程安全 thread-safe
单例

package com.cn.mark.designmode;

public enum SingleEnum {

INSTANCE {

@Override
protected void whateverMethod() {
System.out.println("whateverMethod");
}

};

protected abstract void whateverMethod();

}

static instance

abstract class

Enum类源码片断

public abstract class Enum<E extends Enum<E>>
implements Comparable<E>, Serializable {

overwrite object readObject

Enum类源码片断

private void readObject(ObjectInputStream in) throws IOException,
ClassNotFoundException {
throw new InvalidObjectException("can't deserialize enum");
}