我们先看以下代码，不用ThreadLocal会发生什么情况
package com.qjc.thread.threadLocal; import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors; //ThreadLocal顾名思义表示线程的局部变量，及只有当前线程可以访问，自然是线程安全的
public class ThreadLocalTest { private static final SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"); public static class ParseDate implements Runnable {
int i = 0; public ParseDate(int i) {
this.i = i;
} @Override
public void run() {
try {
Date parse = sdf.parse("2018-04-19 15:12:" + i % 60);
System.out.println(i + ":" + parse);
} catch (ParseException e) {
e.printStackTrace();
}
}
} public static void main(String[] args) {
ExecutorService executorService = Executors.newFixedThreadPool(10);
for (int i = 0; i < 1000; i++) {
executorService.execute(new ParseDate(i));
}
executorService.shutdown();
}
}
控制台输出了这么一个异常
很明显是异常出现在线程上，表明这样做是线程不安全的
下面，我们用ThreadLocal，代码如下：
package com.qjc.thread.threadLocal; import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors; //ThreadLocal顾名思义表示线程的局部变量，及只有当前线程可以访问，自然是线程安全的
public class ThreadLocalTest { // private static final SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
static ThreadLocal<SimpleDateFormat> threadLocal = new ThreadLocal<>(); public static class ParseDate implements Runnable {
int i = 0; public ParseDate(int i) {
this.i = i;
} @Override
public void run() {
try {
// 用ThreadLocal
if (threadLocal.get() == null) {
threadLocal.set(new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"));
}
Date parse = threadLocal.get().parse("2018-04-19 15:12:" + i % 60);
// 不用ThreadLocal
// Date parse = sdf.parse("2018-04-19 15:12:" + i % 60);
System.out.println(i + ":" + parse);
} catch (ParseException e) {
e.printStackTrace();
}
}
} public static void main(String[] args) {
ExecutorService executorService = Executors.newFixedThreadPool(10);
for (int i = 0; i < 1000; i++) {
executorService.execute(new ParseDate(i));
}
executorService.shutdown();
}
}
控制台，正常输出，没有异常出现，表明线程安全。
我们分析一下ThreadLocal的实现原理：
public void set(T value) {
Thread t = Thread.currentThread();//获取当前线程对象
ThreadLocalMap map = getMap(t);//拿到线程的ThreadLocalMap，它是Thread内部的成员(Thread类中的)：ThreadLocal.ThreadLocalMap threadLocals = null;
//并将值设入ThreadLocalMap
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);//从此可以看出，ThreadLocalMap的key就是ThreadLocal当前对象，value就是我们设置的值
}
public T get() {
Thread t = Thread.currentThread();//获取当前线程对象
ThreadLocalMap map = getMap(t);//获取当前线程的ThreadLocalMap对象
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);//然后通过将自己作为key取得内部的实际数据
if (e != null) {
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
return setInitialValue();
}