单例模式和多线程
1.立即加载
public class MyObject {
// 立即加载方式==饿汉模式
private static MyObject myObject = new MyObject();
private MyObject() {
}
public static MyObject getInstance() {
// 此代码版本为立即加载
// 此版本代码的缺点是不能有其它实例变量
// 因为getInstance()方法没有同步
// 所以有可能出现非线程安全问题
return myObject;
}
}
public class MyThread extends Thread {
@Override
public void run() {
System.out.println(MyObject.getInstance().hashCode());
}
}
public class Run {
public static void main(String[] args) {
MyThread t1 = new MyThread();
MyThread t2 = new MyThread();
MyThread t3 = new MyThread();
t1.start();
t2.start();
t3.start();
}
}
运行结果：
23945429
23945429
23945429
2.延迟加载
2.1 延迟加载不同步在多线程环境下的问题
public class MyObject {
private static MyObject myObject;
private MyObject() {
}
public static MyObject getInstance() {
try {
if (myObject != null) {
} else {
// 模拟在创建对象之前做一些准备性的工作
Thread.sleep(3000);
myObject = new MyObject();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
return myObject;
}
}
public class MyThread extends Thread {
@Override
public void run() {
System.out.println(MyObject.getInstance().hashCode());
}
}
public class Run {
public static void main(String[] args) {
MyThread t1 = new MyThread();
MyThread t2 = new MyThread();
MyThread t3 = new MyThread();
t1.start();
t2.start();
t3.start();
}
}
运行结果：
23945429
1668495
3729266
2.2延迟加载的解决方案
1.getInstance方法加锁
2.代码块加锁
3.重要代码加锁（判断和创建那块加锁）
4.使用DCL双检查机制
public class MyObject {
private volatile static MyObject myObject;
private MyObject() {
}
// 使用双检测机制来解决问题
// 即保证了不需要同步代码的异步
// 又保证了单例的效果
public static MyObject getInstance() {
try {
if (myObject != null) {
} else {
// 模拟在创建对象之前做一些准备性的工作
Thread.sleep(3000);
synchronized (MyObject.class) {
if (myObject == null) {
myObject = new MyObject();
}
}
}
} catch (InterruptedException e) {
e.printStackTrace();
}
return myObject;
}
// 此版本的代码称为：
// 双重检查Double-Check Locking
}
public class MyThread extends Thread {
@Override
public void run() {
System.out.println(MyObject.getInstance().hashCode());
}
}
public class Run {
public static void main(String[] args) {
MyThread t1 = new MyThread();
MyThread t2 = new MyThread();
MyThread t3 = new MyThread();
t1.start();
t2.start();
t3.start();
}
}
3.使用静态内部类实现单例模式
public class MyObject {
// 内部类方式
private static class MyObjectHandler {
private static MyObject myObject = new MyObject();
}
private MyObject() {
}
public static MyObject getInstance() {
return MyObjectHandler.myObject;
}
}
public class MyThread extends Thread {
@Override
public void run() {
System.out.println(MyObject.getInstance().hashCode());
}
}
public class Run {
public static void main(String[] args) {
MyThread t1 = new MyThread();
MyThread t2 = new MyThread();
MyThread t3 = new MyThread();
t1.start();
t2.start();
t3.start();
}
}
4.序列化与反序列化单例的实现
public class MyObject implements Serializable {
private static final long serialVersionUID = 888L;
// 内部类方式
private static class MyObjectHandler {
private static final MyObject myObject = new MyObject();
}
private MyObject() {
}
public static MyObject getInstance() {
return MyObjectHandler.myObject;
}
protected Object readResolve() throws ObjectStreamException {
System.out.println("调用了readResolve方法！");
return MyObjectHandler.myObject;
}
}
public class SaveAndRead {
public static void main(String[] args) {
try {
MyObject myObject = MyObject.getInstance();
FileOutputStream fosRef = new FileOutputStream(new File(
"myObjectFile.txt"));
ObjectOutputStream oosRef = new ObjectOutputStream(fosRef);
oosRef.writeObject(myObject);
oosRef.close();
fosRef.close();
System.out.println(myObject.hashCode());
} catch (FileNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
try {
FileInputStream fisRef = new FileInputStream(new File(
"myObjectFile.txt"));
ObjectInputStream iosRef = new ObjectInputStream(fisRef);
MyObject myObject = (MyObject) iosRef.readObject();
iosRef.close();
fisRef.close();
System.out.println(myObject.hashCode());
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
}
}
5.使用static代码块实现单例模式
public class MyObject {
private static MyObject instance = null;
private MyObject() {
}
static {
instance = new MyObject();
}
public static MyObject getInstance() {
return instance;
}
}
public class MyThread extends Thread {
@Override
public void run() {
for (int i = 0; i < 5; i++) {
System.out.println(MyObject.getInstance().hashCode());
}
}
}
public class Run {
public static void main(String[] args) {
MyThread t1 = new MyThread();
MyThread t2 = new MyThread();
MyThread t3 = new MyThread();
t1.start();
t2.start();
t3.start();
}
}
6.使用enum实现单例模式
public class MyObject {
public enum MyEnumSingleton {
connectionFactory;
private Connection connection;
private MyEnumSingleton() {
try {
System.out.println("创建MyObject对象");
String url = "jdbc:sqlserver://localhost:1079;databaseName=y2";
String username = "sa";
String password = "";
String driverName = "com.microsoft.sqlserver.jdbc.SQLServerDriver";
Class.forName(driverName);
connection = DriverManager.getConnection(url, username,
password);
} catch (ClassNotFoundException e) {
e.printStackTrace();
} catch (SQLException e) {
e.printStackTrace();
}
}
public Connection getConnection() {
return connection;
}
}
public static Connection getConnection() {
return MyEnumSingleton.connectionFactory.getConnection();
}
}
public class MyThread extends Thread {
@Override
public void run() {
for (int i = 0; i < 5; i++) {
System.out.println(MyObject.getConnection().hashCode());
}
}
}
public class Run {
public static void main(String[] args) {
MyThread t1 = new MyThread();
MyThread t2 = new MyThread();
MyThread t3 = new MyThread();
t1.start();
t2.start();
t3.start();
}
}
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