# 定义
# 单例是一种创建型设计模式， 让你能够保证一个类只有一个实例， 并提供一个访问该实例的全局节点。
# 结构
# 示例
# 1. 基础单例(单线程): singleton.Singleton.java
实现一个粗糙的单例非常简单。 你仅需隐藏构造函数并实现一个静态的构建方法即可。
多线程调用会出错
package singleton;
public class Singleton {
private static Singleton instance;
public String value;
private Singleton(String value) {
// The following code emulates slow initialization.
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
this.value = value;
}
public static Singleton getInstance(String value) {
if (instance != null) {
return instance;
}
instance = new Singleton(value);
return instance;
}
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
# 2. 单线程中调用基础单例: singleton.Demo.java
package singleton;
public class Demo {
public static void main(String[] args) {
Singleton singleton = Singleton.getInstance("FOO");
Singleton singleton1 = Singleton.getInstance("BAR");
System.out.println(singleton.value);
System.out.println(singleton1.value);
}
}
1
2
3
4
5
6
7
8
9
10
11
12
13
2
3
4
5
6
7
8
9
10
11
12
13
输出
FOO
FOO
1
2
2
# 3. 多线程中调用基础单例: singleton.MultiDemo.java
package singleton;
public class MultiDemo {
public static void main(String[] args) {
Thread threadFoo = new Thread(new ThreadFoo());
Thread threadBar = new Thread(new ThreadBar());
threadFoo.start();
threadBar.start();
}
static class ThreadFoo implements Runnable {
public void run() {
Singleton singleton = Singleton.getInstance("FOO");
System.out.println(singleton.value);
}
}
static class ThreadBar implements Runnable{
public void run() {
Singleton singleton = Singleton.getInstance("BAR");
System.out.println(singleton.value);
}
}
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
输出
BAR
FOO
1
2
2
# 4. 线程安全的单例: singleton.MultiThreadSingleton.java
package singleton;
public class MultiThreadSingleton {
private static volatile MultiThreadSingleton instance;
public String value;
private MultiThreadSingleton(String val) {
this.value = val;
}
public static MultiThreadSingleton getInstance(String value) {
MultiThreadSingleton result = instance;
if (result != null) {
return result;
}
synchronized (MultiThreadSingleton.class) {
if (instance == null) {
instance = new MultiThreadSingleton(value);
}
return instance;
}
}
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
# 5. 多线程中调用线程安全的单例: singleton.MultiDemo1.java
package singleton;
public class MultiDemo1 {
public static void main(String[] args) {
Thread threadFoo = new Thread(new MultiDemo1.ThreadFoo());
Thread threadBar = new Thread(new MultiDemo1.ThreadBar());
threadFoo.start();
threadBar.start();
}
static class ThreadFoo implements Runnable {
public void run() {
MultiThreadSingleton singleton = MultiThreadSingleton.getInstance("FOO");
System.out.println(singleton.value);
}
}
static class ThreadBar implements Runnable{
public void run() {
MultiThreadSingleton singleton = MultiThreadSingleton.getInstance("BAR");
System.out.println(singleton.value);
}
}
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
输出
FOO
FOO
1
2
2