1.
//饿汉式单例
// 它是在类加载的时候就立即初始化，并且创建单例对象
//优点：没有加任何的锁、执行效率比较高，
//在用户体验上来说，比懒汉式更好
//缺点：类加载的时候就初始化，不管你用还是不用，我都占着空间
//浪费了内存，有可能占着茅坑不拉屎
//绝对线程安全，在线程还没出现以前就是实例化了，不可能存在访问安全问题
public class HungrySingleton {
//先静态、后动态
//先属性、后方法
//先上后下
private static final HungrySingleton hungrySingleton = new HungrySingleton();
private HungrySingleton(){}
public static HungrySingleton getInstance(){
return hungrySingleton;
}
}
//饿汉式静态块单例
public class HungryStaticSingleton {
private static final HungryStaticSingleton hungrySingleton;
static {
hungrySingleton = new HungryStaticSingleton();
}
private HungryStaticSingleton(){}
public static HungryStaticSingleton getInstance(){
return hungrySingleton;
}
}
test
public class ConcurrentExecutor {
/**
* @param runHandler
* @param executeCount 发起请求总数
* @param concurrentCount 同时并发执行的线程数
* @throws Exception
*/
public static void execute(final RunHandler runHandler,int executeCount,int concurrentCount) throws Exception {
ExecutorService executorService = Executors.newCachedThreadPool();
//控制信号量，此处用于控制并发的线程数
final Semaphore semaphore = new Semaphore(concurrentCount);
//闭锁，可实现计数量递减
final CountDownLatch countDownLatch = new CountDownLatch(executeCount);
for (int i = 0; i < executeCount; i ++){
executorService.execute(new Runnable() {
public void run() {
try{
//执行此方法用于获取执行许可，当总计未释放的许可数不超过executeCount时,
//则允许同性，否则线程阻塞等待，知道获取到许可
semaphore.acquire();
runHandler.handler();
//释放许可
semaphore.release();
}catch (Exception e){
e.printStackTrace();
}
countDownLatch.countDown();
}
});
}
countDownLatch.await();//线程阻塞，知道闭锁值为0时，阻塞才释放，继续往下执行
executorService.shutdown();
}
public interface RunHandler{
void handler();
}
}
2.
public class LazyDoubleCheckSingleton {
private volatile static LazyDoubleCheckSingleton lazy = null;
private LazyDoubleCheckSingleton(){}
public static LazyDoubleCheckSingleton getInstance(){
if(lazy == null){
synchronized (LazyDoubleCheckSingleton.class){
if(lazy == null){
lazy = new LazyDoubleCheckSingleton();
//1.分配内存给这个对象
//2.初始化对象
//3.设置lazy指向刚分配的内存地址
//4.初次访问对象
}
}
}
return lazy;
}
}
test
//懒汉式单例
//这种形式兼顾饿汉式的内存浪费，也兼顾synchronized性能问题
//完美地屏蔽了这两个缺点
//史上最牛B的单例模式的实现方式
public class LazyInnerClassSingleton {
//默认使用LazyInnerClassGeneral的时候，会先初始化内部类
//如果没使用的话，内部类是不加载的
private LazyInnerClassSingleton(){
if(LazyHolder.LAZY != null){
throw new RuntimeException("不允许创建多个实例");
}
}
//每一个关键字都不是多余的
//static 是为了使单例的空间共享
//保证这个方法不会被重写，重载
public static final LazyInnerClassSingleton getInstance(){
//在返回结果以前，一定会先加载内部类
return LazyHolder.LAZY;
}
//默认不加载
private static class LazyHolder{
private static final LazyInnerClassSingleton LAZY = new LazyInnerClassSingleton();
}
}
test
public class LazyInnerClassSingletonTest {
public static void main(String[] args) {
try{
//很无聊的情况下，进行破坏
Class<?> clazz = LazyInnerClassSingleton.class;
//通过反射拿到私有的构造方法
Constructor c = clazz.getDeclaredConstructor(null);
//强制访问，强吻，不愿意也要吻
c.setAccessible(true);
//暴力初始化
Object o1 = c.newInstance();
//调用了两次构造方法，相当于new了两次
//犯了原则性问题，
Object o2 = c.newInstance();
System.out.println(o1 == o2);
// Object o2 = c.newInstance();
}catch (Exception e){
e.printStackTrace();
}
}
}
//懒汉式单例
//在外部需要使用的时候才进行实例化
public class LazySimpleSingleton {
private LazySimpleSingleton(){}
//静态块，公共内存区域
private static LazySimpleSingleton lazy = null;
public synchronized static LazySimpleSingleton getInstance(){
if(lazy == null){
lazy = new LazySimpleSingleton();
}
return lazy;
}
}
test
public class ExectorThread implements Runnable{
@Override
public void run() {
LazySimpleSingleton singleton = LazySimpleSingleton.getInstance();
// ThreadLocalSingleton singleton = ThreadLocalSingleton.getInstance();
System.out.println(Thread.currentThread().getName() + ":" + singleton);
}
}
public class LazySimpleSingletonTest {
public static void main(String[] args) {
Thread t1 = new Thread(new ExectorThread());
Thread t2 = new Thread(new ExectorThread());
t1.start();
t2.start();
System.out.println("End");
}
}
3.
//Spring中的做法，就是用这种注册式单例
public class ContainerSingleton {
private ContainerSingleton(){}
private static Map<String,Object> ioc = new ConcurrentHashMap<String,Object>();
public static Object getInstance(String className){
synchronized (ioc) {
if (!ioc.containsKey(className)) {
Object obj = null;
try {
obj = Class.forName(className).newInstance();
ioc.put(className, obj);
} catch (Exception e) {
e.printStackTrace();
}
return obj;
} else {
return ioc.get(className);
}
}
}
}
//常量中去使用，常量不就是用来大家都能够共用吗？
//通常在通用API中使用
public enum EnumSingleton {
INSTANCE;
private Object data;
public Object getData() {
return data;
}
public void setData(Object data) {
this.data = data;
}
public static EnumSingleton getInstance(){
return INSTANCE;
}
}
test
public class EnumSingletonTest {
// public static void main(String[] args) {
// try {
// EnumSingleton instance1 = null;
//
// EnumSingleton instance2 = EnumSingleton.getInstance();
// instance2.setData(new Object());
//
// FileOutputStream fos = new FileOutputStream("EnumSingleton.obj");
// ObjectOutputStream oos = new ObjectOutputStream(fos);
// oos.writeObject(instance2);
// oos.flush();
// oos.close();
//
// FileInputStream fis = new FileInputStream("EnumSingleton.obj");
// ObjectInputStream ois = new ObjectInputStream(fis);
// instance1 = (EnumSingleton) ois.readObject();
// ois.close();
//
// System.out.println(instance1.getData());
// System.out.println(instance2.getData());
// System.out.println(instance1.getData() == instance2.getData());
//
// }catch (Exception e){
// e.printStackTrace();
// }
// }
public static void main(String[] args) {
try {
Class clazz = EnumSingleton.class;
Constructor c = clazz.getDeclaredConstructor(String.class,int.class);
c.setAccessible(true);
EnumSingleton enumSingleton = (EnumSingleton)c.newInstance("Tom",666);
}catch (Exception e){
e.printStackTrace();
}
}
}
4.
//反序列化时导致单例破坏
public class SeriableSingleton implements Serializable {
//序列化就是说把内存中的状态通过转换成字节码的形式
//从而转换一个IO流，写入到其他地方(可以是磁盘、网络IO)
//内存中状态给永久保存下来了
//反序列化
//讲已经持久化的字节码内容，转换为IO流
//通过IO流的读取，进而将读取的内容转换为Java对象
//在转换过程中会重新创建对象new
public final static SeriableSingleton INSTANCE = new SeriableSingleton();
private SeriableSingleton(){}
public static SeriableSingleton getInstance(){
return INSTANCE;
}
private Object readResolve(){
return INSTANCE;
}
}
test
public class SeriableSingletonTest {
public static void main(String[] args) {
SeriableSingleton s1 = null;
SeriableSingleton s2 = SeriableSingleton.getInstance();
FileOutputStream fos = null;
try {
fos = new FileOutputStream("SeriableSingleton.obj");
ObjectOutputStream oos = new ObjectOutputStream(fos);
oos.writeObject(s2);
oos.flush();
oos.close();
FileInputStream fis = new FileInputStream("SeriableSingleton.obj");
ObjectInputStream ois = new ObjectInputStream(fis);
s1 = (SeriableSingleton)ois.readObject();
ois.close();
System.out.println(s1);
System.out.println(s2);
System.out.println(s1 == s2);
} catch (Exception e) {
e.printStackTrace();
}
}
}
5.
public class ThreadLocalSingleton {
private static final ThreadLocal<ThreadLocalSingleton> threadLocalInstance =
new ThreadLocal<ThreadLocalSingleton>(){
@Override
protected ThreadLocalSingleton initialValue() {
return new ThreadLocalSingleton();
}
};
private ThreadLocalSingleton(){}
public static ThreadLocalSingleton getInstance(){
return threadLocalInstance.get();
}
}
test
public class ThreadLocalSingletonTest {
public static void main(String[] args) {
System.out.println(ThreadLocalSingleton.getInstance());
System.out.println(ThreadLocalSingleton.getInstance());
System.out.println(ThreadLocalSingleton.getInstance());
System.out.println(ThreadLocalSingleton.getInstance());
System.out.println(ThreadLocalSingleton.getInstance());
Thread t1 = new Thread(new ExectorThread());
Thread t2 = new Thread(new ExectorThread());
t1.start();
t2.start();
System.out.println("End");
}
}
6.
public class Pojo { }
public class ContainerSingletonTest {
public static void main(String[] args) {
try {
long start = System.currentTimeMillis();
ConcurrentExecutor.execute(new ConcurrentExecutor.RunHandler() {
public void handler() {
Object obj = ContainerSingleton.getInstance("com.gupaoedu.vip.pattern.singleton.test.Pojo");;
System.out.println(System.currentTimeMillis() + ": " + obj);
}
}, 10,6);
long end = System.currentTimeMillis();
System.out.println("总耗时：" + (end - start) + " ms.");
}catch (Exception e){
e.printStackTrace();
}
}
}