SimpleDateFormat非线程安全问题
实现1000个线程的时间格式化
package SimpleDateFormat;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.concurrent.LinkedBlockingDeque;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
public class SimpleDateFormat1 {
private static SimpleDateFormat simpleDateFormat = new SimpleDateFormat("mm:ss");
public static void main(String[] args) {
ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(10,10,100,
TimeUnit.MILLISECONDS,new LinkedBlockingDeque<>(1000),new ThreadPoolExecutor.DiscardPolicy());
for (int i = 0; i < 1001; i++) {
int finalI = i;
threadPoolExecutor.submit(new Runnable() {
@Override
public void run() {
Date date = new Date(finalI * 1000);
myFormatTime(date);
}
});
}
threadPoolExecutor.shutdown();
}
private static void myFormatTime(Date date){
System.out.println(simpleDateFormat.format(date));
}
}
产生了线程不安全的问题👇:
这是因为:
多线程的情况下:
线程1在时间片用完之后,线程2来setTime()那么线程1的得到了线程2的时间。
所以可以使用加锁的操作:
就不会有重复的时间了
但是虽然可以解决线程不安全的问题,但是排队等待锁,性能就会变得低
所以可以使用局部变量:
也解决了线程不安全的问题:
但是每次也都会创建新的私有变量
那么有没有一种方案既可以避免加锁排队执行,又不会每次创建任务的时候不会创建私有的变量呢?
那就是ThreadLocal👇:
ThreadLocal
ThreadLocal的作用就是让每一个线程都拥有自己的变量。
那么选择锁还是ThreadLocal?
看创建实列对象的复用率,如果复用率比较高的话,就使用ThreadLocal。
ThreadLocal的原理
类ThreadLocal的主要作用就是将数据放到当前对象的Map中,这个Map时thread类的实列变量。类ThreadLocal自己不管理、不存储任何的数据,它只是数据和Map之间的桥梁。
执行的流程:数据—>ThreadLocal—>currentThread()—>Map。
执行后每个Map存有自己的数据,Map中的key中存储的就是ThreadLocal对象,value就是存储的值。每个Thread的Map值只对当前的线程可见,其它的线程不可以访问当前线程对象中Map的值。当前的线程被销毁,Map也随之被销毁,Map中的数据如果没有被引用、没有被使用,则随时GC回收。
ThreadLocal常用方法
set(T):将内容存储到ThreadLocal
get():从线程去私有的变量
remove():从线程中移除私有变量
package ThreadLocalDemo;
import java.text.SimpleDateFormat;
public class ThreadLocalDemo1 {
private static ThreadLocal<SimpleDateFormat> threadLocal = new ThreadLocal<>();
public static void main(String[] args) {
//设置私有变量
threadLocal.set(new SimpleDateFormat("mm:ss"));
//得到ThreadLocal
SimpleDateFormat simpleDateFormat = threadLocal.get();
//移除
threadLocal.remove();
}
}
ThreadLocal的初始化
ThreadLocal提供了两种初始化的方法
initialValue()和
initialValue()初始化:
package ThreadLocalDemo;
import java.text.SimpleDateFormat;
import java.util.Date;
public class ThreadLocalDemo2 {
//创建并初始化ThreadLocal
private static ThreadLocal<SimpleDateFormat> threadLocal = new ThreadLocal() {
@Override
protected SimpleDateFormat initialValue() {
System.out.println(Thread.currentThread().getName() + "执行了自己的threadLocal中的初始化方法initialValue()");
return new SimpleDateFormat("mm:ss");
}
};
public static void main(String[] args) {
Thread thread1 = new Thread(() -> {
Date date = new Date(5000);
System.out.println("thread0格式化时间之后得结果时:" + threadLocal.get().format(date));
});
thread1.setName("thread0");
thread1.start();
Thread thread2 = new Thread(() -> {
Date date = new Date(6000);
System.out.println("thread1格式化时间之后得结果时:" + threadLocal.get().format(date));
});
thread2.setName("thread1");
thread2.start();
}
}
withInitial方法初始化:
package ThreadLocalDemo;
import java.util.function.Supplier;
public class ThreadLocalDemo3 {
private static ThreadLocal<String> stringThreadLocal =
ThreadLocal.withInitial(new Supplier<String>() {
@Override
public String get() {
System.out.println("执行了withInitial()方法");
return "我是" + Thread.currentThread().getName() + "的ThreadLocal";
}
});
public static void main(String[] args) {
Thread thread1 = new Thread(() -> {
System.out.println(stringThreadLocal.get());
});
thread1.start();
Thread thread2 = new Thread(new Runnable() {
@Override
public void run() {
System.out.println(stringThreadLocal.get());
}
});
thread2.start();
}
}
注意:
ThreadLocal如果使用了set()方法的话,那么它的初始化方法就不会起作用了。
来看:👇
package ThreadLocalDemo;
class Tools {
public static ThreadLocal t1 = new ThreadLocal();
}
class ThreadA extends Thread {
@Override
public void run() {
for (int i = 0; i < 10; i++) {
System.out.println("在ThreadA中取值:" + Tools.t1.get());
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
public class ThreadLocalDemo4 {
public static void main(String[] args) throws InterruptedException {
//main是ThreadA 的 父线程 让main线程set,ThreadA,是get不到的
if (Tools.t1.get() == null) {
Tools.t1.set("main父线程的set");
}
System.out.println("main get 到了: " + Tools.t1.get());
Thread.sleep(1000);
ThreadA a = new ThreadA();
a.start();
}
}
类ThreadLocal不能实现值的继承,那么就可以使用InheritableThreadLocal了👇
InheritableThreadLocal的使用
使用InheritableThreadLocal可以使子线程继承父线程的值
在来看运行的结果:
子线程有最新的值,父线程依旧是旧的值
package ThreadLocalDemo;
class ThreadB extends Thread{
@Override
public void run() {
for (int i = 0; i < 10; i++) {
System.out.println("在ThreadB中取值:" + Tools.t1.get());
if (i == 5){
Tools.t1.set("我是ThreadB中新set()");
}
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
public class ThreadLocalDemo5 {
public static void main(String[] args) throws InterruptedException {
if (Tools.t1.get() == null) {
Tools.t1.set("main父线程的set");
}
System.out.println("main get 到了: " + Tools.t1.get());
Thread.sleep(1000);
ThreadA a = new ThreadA();
a.start();
Thread.sleep(5000);
for (int i = 0; i < 10; i++) {
System.out.println("main的get是:" + Tools.t1.get());
Thread.sleep(100);
}
}
}
ThreadLocal的脏读问题来看👇
package ThreadLocalDemo;
import java.util.concurrent.LinkedBlockingDeque;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
public class ThreadLocalDemo6 {
private static ThreadLocal<String> threadLocal = new ThreadLocal<>();
private static class MyThread extends Thread {
private static boolean flag = false;
@Override
public void run() {
String name = this.getName();
if (!flag) {
threadLocal.set(name);
System.out.println(name + "设置了" + name);
flag = true;
}
System.out.println(name + "得到了" + threadLocal.get());
}
}
public static void main(String[] args) {
ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(1, 1, 0,
TimeUnit.MILLISECONDS, new LinkedBlockingDeque<>(10));
for (int i = 0; i < 2; i++) {
threadPoolExecutor.execute(new MyThread());
}
threadPoolExecutor.shutdown();
}
}
发生了脏读:
线程池复用了线程,也复用了这个线程相关的静态属性,就导致了脏读
那么如何避免脏读呢?
去掉static 之后:
总结
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