194 lines
4.6 KiB
Markdown
194 lines
4.6 KiB
Markdown
# Java 单例设计模式详解
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参考自慕课网课程：[Java设计模式精讲](https://coding.imooc.com/class/270.html):star::star::star::star::star:
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#### 1、单例模式的饿汉模式（线程安全）
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缺点：在类初始化时候就会初始化对应的单例类，如果单例类没有被用到且单例类初始化复杂，就不应用这种模式
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```java
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public class HungrySingleton {
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private static final HungrySingleton hungrySingleton;
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private HungrySingleton(){
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// 如果不加上这个判断，就会被反射攻击
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if (hungrySingleton!=null){
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throw new RuntimeException("单例模式禁止反射调用！");
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}
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}
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static {
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hungrySingleton=new HungrySingleton();
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}
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public static HungrySingleton getInstance(){
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return hungrySingleton;
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}
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}
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```
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```java
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// 反射攻击
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public class ReflectAttack {
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public static void main(String[] args) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
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// 反射攻击
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Class<HungrySingleton> hungrySingletonClass = HungrySingleton.class;
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Constructor<HungrySingleton> constructor = hungrySingletonClass.getDeclaredConstructor();
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// 获取可操作权限
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constructor.setAccessible(true);
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HungrySingleton hungrySingleton = constructor.newInstance();
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HungrySingleton instance = HungrySingleton.getInstance();
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System.out.println(hungrySingleton==instance);
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}
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}
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```
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#### 2.单例模式的懒汉模式（线程不安全）
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```java
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public class LazySingleton {
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private static LazySingleton lazySingleton=null;
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private LazySingleton(){
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if (lazySingleton!=null){
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throw new RuntimeException("单例模式禁止反射调用！");
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}
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}
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// 在多线程下是不安全的
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public static LazySingleton getInstance(){
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if (lazySingleton!=null){
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lazySingleton=new LazySingleton();
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}
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return lazySingleton;
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}
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}
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```
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#### 3.单例模式的懒汉模式（采用双重检查锁，线程安全）
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```java
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public class LazySingleton {
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// 必须要声明为 volatile 防止指令重排序
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private static volatile LazySingleton lazySingleton = null;
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private LazySingleton() {
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if (lazySingleton != null) {
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throw new RuntimeException("单例模式禁止反射调用！");
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}
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}
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public static LazySingleton getInstance() {
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if (lazySingleton == null) {
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synchronized (LazySingleton.class) {
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if (lazySingleton == null) {
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/*
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new对象过程：
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1.分配内存给这个对象
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2.初始化对象
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3.设置lazyDoubleCheckSingleton 指向刚分配的内存地址
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上述三步会发生指令重排序
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*/
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lazySingleton = new LazySingleton();
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}
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}
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}
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return lazySingleton;
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}
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}
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```
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#### 4.使用枚举实现单例（推荐 JVM保证下的单例）
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**能够防止反射攻击和序列化对单例的破坏**
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```java
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// 枚举单例模式
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public enum EnumInstance {
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INSTANCE;
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public static EnumInstance getInstance() {
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return INSTANCE;
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}
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}
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```
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```java
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// 测试
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public class Test {
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public static void main(String[] args) {
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EnumInstance instance0 = EnumInstance.INSTANCE;
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EnumInstance instance1 = EnumInstance.getInstance();
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EnumInstance instance2 = EnumInstance.getInstance();
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System.out.println(instance0 == instance1); // true
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System.out.println(instance0 == instance2); // true
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System.out.println(instance1 == instance2); // true
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}
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}
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```
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```java
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public enum EnumInstance {
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INSTANCE;
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private String data;
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// 可以增加对应的方法
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public void setData(String data) {
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this.data = data;
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}
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public String getData() {
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return data;
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}
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public static EnumInstance getInstance() {
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return INSTANCE;
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}
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}
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```
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#### 5.容器式单例（不常用）
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```java
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public class ContainerSingleton {
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private ContainerSingleton(){
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}
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private static Map<String,Object> singletonMap = new HashMap<String,Object>();
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public static void putInstance(String key,Object instance){
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if(StringUtils.isNotBlank(key) && instance != null){
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if(!singletonMap.containsKey(key)){
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singletonMap.put(key,instance);
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}
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}
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}
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public static Object getInstance(String key){
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return singletonMap.get(key);
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}
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}
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```
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