java基础，单例总结

一，饱汉

public class SingleTon {private SingleTon(){}private final static SingleTon instance = new SingleTon();public static SingleTon getInstance(){return instance;}public static void main(String[] args) {System.out.println("Main方法");}}

或

public class SingleTon {private SingleTon(){}public final static SingleTon instance = new SingleTon();}

二，饿汉，延迟初始化

public class SingleTon2 {private SingleTon2() {System.out.println("构造方法");}private static SingleTon2 instance = null;public static synchronized SingleTon2 getInstance() {if (instance == null) {instance = new SingleTon2();}return instance;}}

二.二  也算是饿汉，但是减少synchronized性能开销的同时，避免double-checked locking

public class SingleTon4 {private static volatile SingleTon4 instance = null;private SingleTon4(){}public static SingleTon4 getInstance(){if(instance == null){synchronized (SingleTon4.class) {if(instance == null){instance = new SingleTon4();}}}return instance;}}

三，基于类初始化的解决方案。线程安全的延迟初始化方案

JVM在类的初始化阶段（即在Class被加载后，且被线程使用之前），会执行类的初始化。
在执行类的初始化期间，JVM会去获取一个锁。这个锁可以同步多个线程对同一个类的初始化。
基于这个特性，可以实现另一种线程安全的延迟初始化方案（这个方案被称之为Initialization On Demand Holder idiom）

public class SingleTon5 {private SingleTon5(){System.out.println("heihi");}private static class InstanceHolder {public static SingleTon5 instance = new SingleTon5();}public static SingleTon5 getInstance() {return InstanceHolder.instance ;  //这里将导致InstanceHolder类被初始化}}

四，枚举类实现

/** * 单元素的枚举类型是实现单例的最佳方法 */public enum SingleEnum {INSTANCE;public void sayHello(){System.err.println("Hello me");}}

五，记录式单例

jkd8

public class SingleTon3_3 {private static Map<String, SingleTon3_3> map = new HashMap<String, SingleTon3_3>();private SingleTon3_3(String name) {}public static SingleTon3_3 getInstance(String name) {name = Objects.toString(name, SingleTon3_3.class.getName());return map.computeIfAbsent(name, SingleTon3_3::new);}}

jdk7

/** * 记录式单例 */public class SingleTon3 {private static Map<String, SingleTon3> map = new HashMap<String, SingleTon3>();static {SingleTon3 single = new SingleTon3();map.put(single.getClass().getName(), single);}private SingleTon3() {System.out.println("构造method");}public static SingleTon3 getInstance(String name) {if (name == null) {name = SingleTon3.class.getName();}if (map.get(name) == null) {System.out.println("得到" + name);try {map.put(name, (SingleTon3) Class.forName("single_ton.SingleTon3").newInstance());} catch (InstantiationException e) {e.printStackTrace();} catch (IllegalAccessException e) {e.printStackTrace();} catch (ClassNotFoundException e) {e.printStackTrace();}}return map.get(name);}}