单例模式

单例模式

饿汉式

//饿汉式单例
public class Hungry {
​
    //可能浪费空间
    private byte[] data1 =new byte[1024*1024];
    private byte[] data2 =new byte[1024*1024];
    private byte[] data3 =new byte[1024*1024];
    private byte[] data4 =new byte[1024*1024];
​
    private Hungry(){
    }
​
    private final static Hungry HUNGRY=new Hungry();
    public static Hungry getInstance(){
        return HUNGRY;
    }
}

DCL懒汉式

package com.vogt.single;
​
import javafx.scene.control.Label;
​
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
​
/**
 * @author VOGT
 * @version V1.0
 * @Package com.vogt.single
 * @date 2020/4/13 10:34
 */
//懒汉式单例
public class LazyMan {
    public static boolean vogt=false;
    private LazyMan() {
        synchronized (LazyMan.class){
            if(vogt==false){
                vogt=true;
            }else {
                throw new RuntimeException("不要试图用反射破坏异常");
            }
        }
    }
​
    private volatile static LazyMan lazyMan;
​
    public static LazyMan getInstance() {
        //双重检测锁模式的懒汉式单例，DCL懒汉式
        if (lazyMan == null) {
            synchronized (LazyMan.class) {
                if (lazyMan == null) {
                    lazyMan = new LazyMan();//不是一个原子性操作
                    /*
                     * 1.分配内存空间
                     * 2.执行构造方法、初始化对象
                     * 3.把对象指向这个空间
                     *
                     * 123
                     * 132 A
                     *     B//可能lazyMan还没有完成构造
                     * */
                }
            }
        }
        return lazyMan;
    }
    //反射破解
    public static void main(String[] args) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException, NoSuchFieldException {
        //LazyMan instance = LazyMan.getInstance();
        Field vogt = LazyMan.class.getDeclaredField("vogt");
        vogt.setAccessible(true);
​
        Constructor<LazyMan> declaredConstructor = LazyMan.class.getDeclaredConstructor(null);
        declaredConstructor.setAccessible(true);
        LazyMan instance = declaredConstructor.newInstance();
​
        vogt.set(instance,false);
​
        LazyMan instance2 = declaredConstructor.newInstance();
​
        System.out.println(instance);
        System.out.println(instance2);
    }
​
}
//
//    //单线程ok，多线程不行
//    public static void main(String[] args) {
//        for (int i = 0; i < 10; i++) {
//            new Thread(() -> {
//                LazyMan.getInstance();
//            }).start();
//        }
//
//
//    }
//}
​

静态内部类式

package com.vogt.single;
​
/**
 * @author VOGT
 * @version V1.0
 * @Package com.vogt.single
 * @date 2020/4/13 10:47
 */
//静态内部类
public class Holder {
    private Holder(){
​
    }
​
    public static Holder getInstance(){
        return InnereClass.HOLDER;
    }
​
    public static class  InnereClass{
        private static final Holder HOLDER=new Holder();
    }
}
​

单例不安全，反射

枚举

package com.vogt.single;
​
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
​
/**
 * @author VOGT
 * @version V1.0
 * @Package com.vogt.single
 * @date 2020/4/13 11:00
 */
//枚举本身也是一个class类
public enum EnumSingle {
    INSTANCE;
​
    public EnumSingle getInstance(){
     return INSTANCE;
    }
}
class Test{
    public static void main(String[] args) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
        EnumSingle instance1 = EnumSingle.INSTANCE;
​
​
        Constructor<EnumSingle> declaredConstructor = EnumSingle.class.getDeclaredConstructor(String.class,int.class);
        declaredConstructor.setAccessible(true);
        EnumSingle instance2 = declaredConstructor.newInstance();
        //Exception in thread "main" java.lang.NoSuchMethodException: com.vogt.single.EnumSingle.<init>()
        System.out.println(instance1);
        System.out.println(instance2);
​
​
    }
}
​

枚举的（以为）反编译源码：

Compiled from "EnumSingle.java"
public final class com.vogt.single.EnumSingle extends java.lang.Enum<com.vogt.single.EnumSingle> {
  public static final com.vogt.single.EnumSingle INSTANCE;
  private static final com.vogt.single.EnumSingle[] $VALUES;
  public static com.vogt.single.EnumSingle[] values();
  public static com.vogt.single.EnumSingle valueOf(java.lang.String);
  private com.vogt.single.EnumSingle();
  public com.vogt.single.EnumSingle getInstance();
  static {};
}

枚举的最终反编译源码：

​
package com.vogt.single;
​
​
public final class EnumSingle extends Enum
{
​
    public static EnumSingle[] values()
    {
        return (EnumSingle[])$VALUES.clone();
    }
​
    public static EnumSingle valueOf(String name)
    {
        return (EnumSingle)Enum.valueOf(com/vogt/single/EnumSingle, name);
    }
​
    private EnumSingle(String s, int i)
    {
        super(s, i);
    }
​
    public EnumSingle getInstance()
    {
        return INSTANCE;
    }
​
    public static final EnumSingle INSTANCE;
    private static final EnumSingle $VALUES[];
​
    static 
    {
        INSTANCE = new EnumSingle("INSTANCE", 0);
        $VALUES = (new EnumSingle[] {
            INSTANCE
        });
    }
}
​