单例模式

1 单例模式（Singleton）

1.1 定义

保证一个类只有一个实例，并且提供一个全局访问点。

1.2 场景

重量级的对象，不需要多个实例，如线程池、数据库连接池等。

1.3 类图

点击查看源网页

1.4 实现

1、懒汉式

延迟加载，只有在真正使用的时候，才开始实例化。
问题：
- 线程安全问题。
- double check加锁优化。
- 编译器（JIT）、CPU有可能对指令进行重排序，导致使用到尚未初始化的实例。可以通过添加volatile关键字进行修饰。对于volatile修饰的字段，可以防止指令重排。

1）一般方式

package com.xianhuii.designpattern.singleton;

public class LazySingleton {
    private volatile static LazySingleton instance;

    private LazySingleton() {
    }

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

反汇编javap -v LazySingleton.class：

Classfile /D:/exam/out/production/exam/com/xianhuii/designpattern/singleton/LazySingleton.class
  Last modified 2020年5月14日; size 624 bytes
  MD5 checksum 8fe7d4bc5ddd17a3ab812ce299308287
  Compiled from "LazySingleton.java"
public class com.xianhuii.designpattern.singleton.LazySingleton
  minor version: 0
  major version: 55
  flags: (0x0021) ACC_PUBLIC, ACC_SUPER
  this_class: #3                          // com/xianhuii/designpattern/singleton/LazySingleton
  super_class: #5                         // java/lang/Object
  interfaces: 0, fields: 1, methods: 2, attributes: 1
Constant pool:
   #1 = Methodref          #5.#20         // java/lang/Object."<init>":()V
   #2 = Fieldref           #3.#21         // com/xianhuii/designpattern/singleton/LazySingleton.instance:Lcom/xianhuii/designpattern/singleton
/LazySingleton;
   #3 = Class              #22            // com/xianhuii/designpattern/singleton/LazySingleton
   #4 = Methodref          #3.#20         // com/xianhuii/designpattern/singleton/LazySingleton."<init>":()V
   #5 = Class              #23            // java/lang/Object
   #6 = Utf8               instance
   #7 = Utf8               Lcom/xianhuii/designpattern/singleton/LazySingleton;
   #8 = Utf8               <init>
   #9 = Utf8               ()V
  #10 = Utf8               Code
  #11 = Utf8               LineNumberTable
  #12 = Utf8               LocalVariableTable
  #13 = Utf8               this
  #14 = Utf8               getInstance
  #15 = Utf8               ()Lcom/xianhuii/designpattern/singleton/LazySingleton;
  #16 = Utf8               StackMapTable
  #17 = Class              #24            // java/lang/Throwable
  #18 = Utf8               SourceFile
  #19 = Utf8               LazySingleton.java
  #20 = NameAndType        #8:#9          // "<init>":()V
  #21 = NameAndType        #6:#7          // instance:Lcom/xianhuii/designpattern/singleton/LazySingleton;
  #22 = Utf8               com/xianhuii/designpattern/singleton/LazySingleton
  #23 = Utf8               java/lang/Object
  #24 = Utf8               java/lang/Throwable
{
  public static com.xianhuii.designpattern.singleton.LazySingleton getInstance();
    descriptor: ()Lcom/xianhuii/designpattern/singleton/LazySingleton;
    flags: (0x0009) ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=2, args_size=0
         0: getstatic     #2                  // Field instance:Lcom/xianhuii/designpattern/singleton/LazySingleton;
         3: ifnonnull     37
         6: ldc           #3                  // class com/xianhuii/designpattern/singleton/LazySingleton
         8: dup
         9: astore_0
        10: monitorenter
        11: getstatic     #2                  // Field instance:Lcom/xianhuii/designpattern/singleton/LazySingleton;
        14: ifnonnull     27
        17: new           #3                  // class com/xianhuii/designpattern/singleton/LazySingleton
        20: dup
        21: invokespecial #4                  // Method "<init>":()V
        24: putstatic     #2                  // Field instance:Lcom/xianhuii/designpattern/singleton/LazySingleton;
        27: aload_0
        28: monitorexit
        29: goto          37
        32: astore_1
        33: aload_0
        34: monitorexit
        35: aload_1
        36: athrow
        37: getstatic     #2                  // Field instance:Lcom/xianhuii/designpattern/singleton/LazySingleton;
        40: areturn
      Exception table:
         from    to  target type
            11    29    32   any
            32    35    32   any
      LineNumberTable:
        line 10: 0
        line 11: 6
        line 12: 11
        line 13: 17
        line 15: 27
        line 17: 37
      StackMapTable: number_of_entries = 3
        frame_type = 252 /* append */
          offset_delta = 27
          locals = [ class java/lang/Object ]
        frame_type = 68 /* same_locals_1_stack_item */
          stack = [ class java/lang/Throwable ]
        frame_type = 250 /* chop */
          offset_delta = 4
}
SourceFile: "LazySingleton.java"

2）静态内部类方式

本质上借助于JVM的类加载机制，保证实例的唯一性。
只有在实际使用的时候，才会触发类的初始化，所以也是懒加载的一种方式。

package com.xianhuii.designpattern.singleton;

public class InnerClassLazySingleton {
    private InnerClassLazySingleton(){}

    private static class Inner {
        private static InnerClassLazySingleton instance = new InnerClassLazySingleton();
    }

    public static InnerClassLazySingleton getInstance() {
        return Inner.instance;
    }
}

反射攻击：

package com.xianhuii;

import com.xianhuii.designpattern.singleton.InnerClassLazySingleton;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;

public class MyTest {
    public static void main(String[] args) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
        Constructor<InnerClassLazySingleton> cons = InnerClassLazySingleton.class.getDeclaredConstructor();
        cons.setAccessible(true);
        InnerClassLazySingleton instance1 = cons.newInstance();
        InnerClassLazySingleton instance2 = InnerClassLazySingleton.getInstance();
        System.out.println(instance1 == instance2);    // false
    }
}

序列化问题：

package com.xianhuii.designpattern.singleton;

import java.io.*;

public class InnerClassLazySingleton implements Serializable {
    private static final long serialVersionUID = 42L;
    private InnerClassLazySingleton(){}

    private static class Inner {
        private static InnerClassLazySingleton instance = new InnerClassLazySingleton();
    }

    public static InnerClassLazySingleton getInstance() {
        return Inner.instance;
    }

//    private Object readResolve() throws ObjectStreamException {
//        return Inner.instance;
//    }

    public static void main(String[] args) throws Exception {
        ObjectOutputStream output = new ObjectOutputStream(new FileOutputStream(new File("test")));
        InnerClassLazySingleton instance1 = InnerClassLazySingleton.getInstance();
        output.writeObject(instance1);
        ObjectInputStream input = new ObjectInputStream(new FileInputStream(new File("test")));
        InnerClassLazySingleton instance2 = (InnerClassLazySingleton) input.readObject();
        System.out.println(instance1 == instance2); // false
    }
}

解决方式：添加readResolve()方法。

2、饿汉式

类加载的初始化阶段就完成了实例的初始化。
本质上借助于JVM的类加载机制，保证实例的唯一性。
类加载过程：
1. 加载：加载二进制数据到内存中，生成对应的Class数据结构。
2. 连接：验证、准备（给类的静态成员变量赋默认值）、解析。
3. 初始化：给类的静态变量赋初值。
只有在真正使用对应的类时，才会触发初始化：
- 当前类是启动类，即main函数所在类。
- 直接进行new操作。
- 访问类的静态属性、静态方法。
- 使用反射访问类。
- 初始化一个类的子类。

1）一般方式

package com.xianhuii.designpattern.singleton;

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

    private HungrySingleton() {
    }

    public static HungrySingleton getInstance() {
        return instance;
    }
}

2）枚举方式

天然防止反射攻击、序列化问题。

package com.xianhuii.designpattern.singleton;

public enum EnumSingleton {
    INSTANCE;
}

反汇编javap -v -p EnumSingleton.class：

Classfile /D:/exam/out/production/exam/com/xianhuii/designpattern/singleton/EnumSingleton.class
  Last modified 2020年5月14日; size 1072 bytes
  MD5 checksum 7ae8019a05f48d1a3c1ca40eb711180d
  Compiled from "EnumSingleton.java"
public final class com.xianhuii.designpattern.singleton.EnumSingleton extends java.lang.Enum<com.xianhuii.designpattern.singleton.EnumSingleto
n>
  minor version: 0
  major version: 55
  flags: (0x4031) ACC_PUBLIC, ACC_FINAL, ACC_SUPER, ACC_ENUM
  this_class: #4                          // com/xianhuii/designpattern/singleton/EnumSingleton
  super_class: #10                        // java/lang/Enum
  interfaces: 0, fields: 2, methods: 4, attributes: 2
Constant pool:
   #1 = Fieldref           #4.#33         // com/xianhuii/designpattern/singleton/EnumSingleton.$VALUES:[Lcom/xianhuii/designpattern/singleton
/EnumSingleton;
   #2 = Methodref          #34.#35        // "[Lcom/xianhuii/designpattern/singleton/EnumSingleton;".clone:()Ljava/lang/Object;
   #3 = Class              #14            // "[Lcom/xianhuii/designpattern/singleton/EnumSingleton;"
   #4 = Class              #36            // com/xianhuii/designpattern/singleton/EnumSingleton
   #5 = Methodref          #10.#37        // java/lang/Enum.valueOf:(Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum;
   #6 = Methodref          #10.#38        // java/lang/Enum."<init>":(Ljava/lang/String;I)V
   #7 = String             #11            // INSTANCE
   #8 = Methodref          #4.#38         // com/xianhuii/designpattern/singleton/EnumSingleton."<init>":(Ljava/lang/String;I)V
   #9 = Fieldref           #4.#39         // com/xianhuii/designpattern/singleton/EnumSingleton.INSTANCE:Lcom/xianhuii/designpattern/singleton
/EnumSingleton;
  #10 = Class              #40            // java/lang/Enum
  #11 = Utf8               INSTANCE
  #12 = Utf8               Lcom/xianhuii/designpattern/singleton/EnumSingleton;
  #13 = Utf8               $VALUES
  #14 = Utf8               [Lcom/xianhuii/designpattern/singleton/EnumSingleton;
  #15 = Utf8               values
  #16 = Utf8               ()[Lcom/xianhuii/designpattern/singleton/EnumSingleton;
  #17 = Utf8               Code
  #18 = Utf8               LineNumberTable
  #19 = Utf8               valueOf
  #20 = Utf8               (Ljava/lang/String;)Lcom/xianhuii/designpattern/singleton/EnumSingleton;
  #21 = Utf8               LocalVariableTable
  #22 = Utf8               name
  #23 = Utf8               Ljava/lang/String;
  #24 = Utf8               <init>
  #25 = Utf8               (Ljava/lang/String;I)V
  #26 = Utf8               this
  #27 = Utf8               Signature
  #28 = Utf8               ()V
  #29 = Utf8               <clinit>
  #30 = Utf8               Ljava/lang/Enum<Lcom/xianhuii/designpattern/singleton/EnumSingleton;>;
  #31 = Utf8               SourceFile
  #32 = Utf8               EnumSingleton.java
  #33 = NameAndType        #13:#14        // $VALUES:[Lcom/xianhuii/designpattern/singleton/EnumSingleton;
  #34 = Class              #14            // "[Lcom/xianhuii/designpattern/singleton/EnumSingleton;"
  #35 = NameAndType        #41:#42        // clone:()Ljava/lang/Object;
  #36 = Utf8               com/xianhuii/designpattern/singleton/EnumSingleton
  #37 = NameAndType        #19:#43        // valueOf:(Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum;
  #38 = NameAndType        #24:#25        // "<init>":(Ljava/lang/String;I)V
  #39 = NameAndType        #11:#12        // INSTANCE:Lcom/xianhuii/designpattern/singleton/EnumSingleton;
  #40 = Utf8               java/lang/Enum
  #41 = Utf8               clone
  #42 = Utf8               ()Ljava/lang/Object;
  #43 = Utf8               (Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum;
{
  public static final com.xianhuii.designpattern.singleton.EnumSingleton INSTANCE;
    descriptor: Lcom/xianhuii/designpattern/singleton/EnumSingleton;
    flags: (0x4019) ACC_PUBLIC, ACC_STATIC, ACC_FINAL, ACC_ENUM

  private static final com.xianhuii.designpattern.singleton.EnumSingleton[] $VALUES;
    descriptor: [Lcom/xianhuii/designpattern/singleton/EnumSingleton;
    flags: (0x101a) ACC_PRIVATE, ACC_STATIC, ACC_FINAL, ACC_SYNTHETIC

  public static com.xianhuii.designpattern.singleton.EnumSingleton[] values();
    descriptor: ()[Lcom/xianhuii/designpattern/singleton/EnumSingleton;
    flags: (0x0009) ACC_PUBLIC, ACC_STATIC
    Code:
      stack=1, locals=0, args_size=0
         0: getstatic     #1                  // Field $VALUES:[Lcom/xianhuii/designpattern/singleton/EnumSingleton;
         3: invokevirtual #2                  // Method "[Lcom/xianhuii/designpattern/singleton/EnumSingleton;".clone:()Ljava/lang/Object;
         6: checkcast     #3                  // class "[Lcom/xianhuii/designpattern/singleton/EnumSingleton;"
         9: areturn
      LineNumberTable:
        line 3: 0

  public static com.xianhuii.designpattern.singleton.EnumSingleton valueOf(java.lang.String);
    descriptor: (Ljava/lang/String;)Lcom/xianhuii/designpattern/singleton/EnumSingleton;
    flags: (0x0009) ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=1, args_size=1
         0: ldc           #4                  // class com/xianhuii/designpattern/singleton/EnumSingleton
         2: aload_0
         3: invokestatic  #5                  // Method java/lang/Enum.valueOf:(Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum;
         6: checkcast     #4                  // class com/xianhuii/designpattern/singleton/EnumSingleton
         9: areturn
      LineNumberTable:
        line 3: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      10     0  name   Ljava/lang/String;

  private com.xianhuii.designpattern.singleton.EnumSingleton();
    descriptor: (Ljava/lang/String;I)V
    flags: (0x0002) ACC_PRIVATE
    Code:
      stack=3, locals=3, args_size=3
         0: aload_0
         1: aload_1
         2: iload_2
         3: invokespecial #6                  // Method java/lang/Enum."<init>":(Ljava/lang/String;I)V
         6: return
      LineNumberTable:
        line 3: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       7     0  this   Lcom/xianhuii/designpattern/singleton/EnumSingleton;
    Signature: #28                          // ()V

  static {};
    descriptor: ()V
    flags: (0x0008) ACC_STATIC
    Code:
      stack=4, locals=0, args_size=0
         0: new           #4                  // class com/xianhuii/designpattern/singleton/EnumSingleton
         3: dup
         4: ldc           #7                  // String INSTANCE
         6: iconst_0
         7: invokespecial #8                  // Method "<init>":(Ljava/lang/String;I)V
        10: putstatic     #9                  // Field INSTANCE:Lcom/xianhuii/designpattern/singleton/EnumSingleton;
        13: iconst_1
        14: anewarray     #4                  // class com/xianhuii/designpattern/singleton/EnumSingleton
        17: dup
        18: iconst_0
        19: getstatic     #9                  // Field INSTANCE:Lcom/xianhuii/designpattern/singleton/EnumSingleton;
        22: aastore
        23: putstatic     #1                  // Field $VALUES:[Lcom/xianhuii/designpattern/singleton/EnumSingleton;
        26: return
      LineNumberTable:
        line 4: 0
        line 3: 13
}
Signature: #30                          // Ljava/lang/Enum<Lcom/xianhuii/designpattern/singleton/EnumSingleton;>;
SourceFile: "EnumSingleton.java"