ThreadLocal 示例
ThreadLocal, 从字面意思上看是本地线程. 但实际上它是一个线程本地变量.它的功能就是为每一个使用该变量的线程都提供一个变量值的副本, 从而使得不会与其他线程的副本冲突. 与使用synchronized解决同步问题一样的作用, 区别是synchronized是通过使用加锁的方式来实现的,而ThreadLocal是通过其内部定义的一个Map来存放每一个线程的变量副本来实现的.
看下面的例子, 通过多个线程来设置Student的age属性:
package threadLocal;
public class Student {
private int age;
private String name;
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@Override
public String toString() {
return "Student [age=" + age + ", name=" + name + "]";
}
}
有同步问题的代码:
package threadLocal;
import java.util.Random;
public class ThreadDemo implements Runnable {
private static Student stu = new Student();
@Override
public void run() {
accessStudent();
}
private void accessStudent() {
try {
Random r = new Random();
int age =r.nextInt(100);
stu.setAge(age);
System.out.println(Thread.currentThread().getName() + ":" + stu);
Thread.sleep(3000);
System.out.println(Thread.currentThread().getName() + ":" + "after 3 second:" + stu);
} catch (Exception e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
ThreadDemo demo = new ThreadDemo();
for(int i=0;i<10;i++) {
Thread a = new Thread(demo, "a"+i);
a.start();
}
}
} 结果: a4:Student [age=66, name=null]
a0:Student [age=87, name=null]
a1:Student [age=66, name=null]
a2:Student [age=66, name=null]
a3:Student [age=63, name=null]
a5:Student [age=63, name=null]
a8:Student [age=73, name=null]
a9:Student [age=14, name=null]
a6:Student [age=6, name=null]
a7:Student [age=45, name=null]
a3:after 3 second:Student [age=45, name=null]
a1:after 3 second:Student [age=45, name=null]
a0:after 3 second:Student [age=45, name=null]
a4:after 3 second:Student [age=45, name=null]
a9:after 3 second:Student [age=45, name=null]
a8:after 3 second:Student [age=45, name=null]
a5:after 3 second:Student [age=45, name=null]
a7:after 3 second:Student [age=45, name=null]
a2:after 3 second:Student [age=45, name=null]
a6:after 3 second:Student [age=45, name=null]
使用synchronized解决:
package threadLocal;
import java.util.Random;
public class ThreadDemo implements Runnable {
private static Student stu = new Student();
@Override
public void run() {
accessStudent();
}
private synchronized void accessStudent() {
try {
Random r = new Random();
int age =r.nextInt(100);
stu.setAge(age);
System.out.println(Thread.currentThread().getName() + ":" + stu);
Thread.sleep(3000);
System.out.println(Thread.currentThread().getName() + ":" + "after 3 second:" + stu);
} catch (Exception e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
ThreadDemo demo = new ThreadDemo();
for(int i=0;i<10;i++) {
Thread a = new Thread(demo, "a"+i);
a.start();
}
}
} 结果: a0:Student [age=86, name=null]
a0:after 3 second:Student [age=86, name=null]
a5:Student [age=14, name=null]
a5:after 3 second:Student [age=14, name=null]
a4:Student [age=31, name=null]
a4:after 3 second:Student [age=31, name=null]
a8:Student [age=58, name=null]
a8:after 3 second:Student [age=58, name=null]
a9:Student [age=5, name=null]
a9:after 3 second:Student [age=5, name=null]
a6:Student [age=10, name=null]
a6:after 3 second:Student [age=10, name=null]
a7:Student [age=22, name=null]
a7:after 3 second:Student [age=22, name=null]
a2:Student [age=37, name=null]
a2:after 3 second:Student [age=37, name=null]
a3:Student [age=92, name=null]
a3:after 3 second:Student [age=92, name=null]
a1:Student [age=41, name=null]
a1:after 3 second:Student [age=41, name=null]
使用ThreadLocal解决:
package threadLocal;
import java.util.Random;
public class ThreadLocalDemo implements Runnable {
private static ThreadLocal<Student> studentLocal = new ThreadLocal<Student>();
@Override
public void run() {
accessStudent();
}
private void accessStudent() {
Student s = getStudent();
Random r = new Random();
int age = r.nextInt(100);
s.setAge(age);
System.out.println(Thread.currentThread().getName() + ":" + s);
try {
Thread.sleep(3000);
System.out.println(Thread.currentThread().getName() + ":" + "after 3 second:" + s);
} catch (Exception e) {
e.printStackTrace();
}
}
private Student getStudent() {
Student s = studentLocal.get();
if (s == null) {
s = new Student();
studentLocal.set(s);
}
return s;
}
public static void main(String[] args) {
ThreadLocalDemo demo = new ThreadLocalDemo();
for(int i=0;i<10;i++) {
Thread a = new Thread(demo, "a"+i);
a.start();
}
}
}
结果:
a9:Student [age=72, name=null]
a3:Student [age=10, name=null]
a8:Student [age=24, name=null]
a7:Student [age=5, name=null]
a1:Student [age=65, name=null]
a2:Student [age=39, name=null]
a6:Student [age=5, name=null]
a5:Student [age=61, name=null]
a4:Student [age=47, name=null]
a0:Student [age=41, name=null]
a9:after 3 second:Student [age=72, name=null]
a8:after 3 second:Student [age=24, name=null]
a3:after 3 second:Student [age=10, name=null]
a7:after 3 second:Student [age=5, name=null]
a1:after 3 second:Student [age=65, name=null]
a2:after 3 second:Student [age=39, name=null]
a5:after 3 second:Student [age=61, name=null]
a6:after 3 second:Student [age=5, name=null]
a0:after 3 second:Student [age=41, name=null]
a4:after 3 second:Student [age=47, name=null]
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