ThreadLocal使用demo
public class ThreadLocalDemo {
public static ThreadLocal threadName = new ThreadLocal(){
protected String initialValue() {
return "-1";
}
};
public static void main(String[] args) throws InterruptedException {
ThreadLocalDemo.threadName.set("789");
Thread t1 = new Thread(){
public void run() {
ThreadLocalDemo.threadName.set("123");
System.out.println("ThreadLocalDemo.threadName in t1 is : " + ThreadLocalDemo.threadName.get());
ThreadLocalDemo.threadName.remove();
System.out.println("ThreadLocalDemo.threadName in t1 is : " + ThreadLocalDemo.threadName.get());
}
};
Thread t2 = new Thread(){
public void run() {
ThreadLocalDemo.threadName.set("456");
System.out.println("ThreadLocalDemo.threadName in t2 is : " + ThreadLocalDemo.threadName.get());
ThreadLocalDemo.threadName.remove();
System.out.println("ThreadLocalDemo.threadName in t2 is : " + ThreadLocalDemo.threadName.get());
}
};
t1.start();
t1.join();
t2.start();
t2.join();
System.out.println("ThreadLocalDemo.threadName in main is : " + ThreadLocalDemo.threadName.get());
}
}
上边是ThreadLocal的一个使用demo，需要处理的细节是：ThreadLocal使用完后，需要调用remove清除当前线程的变量副本，否则，会产生内存溢出，如果是使用线程池，由于线程是共享的，还会为其他任务留下脏数据。
public class ThreadLocalDemo4Executor {
public static ThreadLocal threadName = new ThreadLocal(){
protected String initialValue() {
return "-1";
}
};
public static void main(String[] args) throws InterruptedException {
ExecutorService executor = Executors.newFixedThreadPool(10);
ThreadLocalDemo.threadName.set("789");
for(int i = 0; i < 20; i++) {
executor.execute(new Thread(){
public void run() {
System.out.println("Before####ThreadLocalDemo.threadName in thread:" + Thread.currentThread() + " is : " + ThreadLocalDemo.threadName.get());
ThreadLocalDemo.threadName.set(new Random().nextInt(100) + "");
System.out.println("After####ThreadLocalDemo.threadName in thread:" + Thread.currentThread() + " is : " + ThreadLocalDemo.threadName.get());
}
} );
}
System.out.println("ThreadLocalDemo.threadName in main is : " + ThreadLocalDemo.threadName.get());
}
}
输出：
"C:\Program Files\Java\jdk1.8.0_60\bin\java" "-javaagent:C:\Program Files\JetBrains\IntelliJ IDEA 2017.2.3\lib\idea_rt.jar=55943:C:\Program Files\JetBrains\IntelliJ IDEA 2017.2.3\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.8.0_60\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_60\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_60\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_60\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_60\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_60\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.8.0_60\jre\lib\ext\jfxrt.jar;C:\Program Files\Java\jdk1.8.0_60\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.8.0_60\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_60\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.8.0_60\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.8.0_60\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.8.0_60\jre\lib\ext\sunpkcs11.jar;C:\Program Files\Java\jdk1.8.0_60\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.8.0_60\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.8.0_60\jre\lib\jce.jar;C:\Program Files\Java\jdk1.8.0_60\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.8.0_60\jre\lib\jfxswt.jar;C:\Program Files\Java\jdk1.8.0_60\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.8.0_60\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.8.0_60\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.8.0_60\jre\lib\resources.jar;C:\Program Files\Java\jdk1.8.0_60\jre\lib\rt.jar;C:\TestProj\billTest\target\classes;C:\EDISK\maven\repo\com\alibaba\fastjson\1.2.38\fastjson-1.2.38.jar" com.blackfish.bill.billlist.ThreadLocalDemo4Executor
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-1,5,main] is : -1
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-2,5,main] is : -1
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-3,5,main] is : -1
ThreadLocalDemo.threadName in main is : 789
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-3,5,main] is : 22
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-2,5,main] is : 13
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-5,5,main] is : -1
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-5,5,main] is : 1
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-5,5,main] is : 1
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-5,5,main] is : 55
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-4,5,main] is : -1
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-4,5,main] is : 82
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-4,5,main] is : 82
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-4,5,main] is : 30
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-4,5,main] is : 30
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-7,5,main] is : -1
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-1,5,main] is : 20
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-7,5,main] is : 49
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-7,5,main] is : 49
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-1,5,main] is : 20
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-4,5,main] is : 79
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-8,5,main] is : -1
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-5,5,main] is : 55
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-3,5,main] is : 22
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-3,5,main] is : 81
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-3,5,main] is : 81
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-2,5,main] is : 13
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-6,5,main] is : -1
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-10,5,main] is : -1
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-2,5,main] is : 45
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-3,5,main] is : 71
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-9,5,main] is : -1
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-5,5,main] is : 12
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-8,5,main] is : 54
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-4,5,main] is : 79
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-1,5,main] is : 67
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-7,5,main] is : 56
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-4,5,main] is : 51
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-9,5,main] is : 44
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-10,5,main] is : 70
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-6,5,main] is : 94
修复：增加一行 ThreadLocalDemo.threadName.remove();
public class ThreadLocalDemo4Executor {
public static ThreadLocal threadName = new ThreadLocal(){
protected String initialValue() {
return "-1";
}
};
public static void main(String[] args) throws InterruptedException {
ExecutorService executor = Executors.newFixedThreadPool(10);
ThreadLocalDemo.threadName.set("789");
for(int i = 0; i < 20; i++) {
executor.execute(new Thread(){
public void run() {
System.out.println("Before####ThreadLocalDemo.threadName in thread:" + Thread.currentThread() + " is : " + ThreadLocalDemo.threadName.get());
ThreadLocalDemo.threadName.set(new Random().nextInt(100) + "");
System.out.println("After####ThreadLocalDemo.threadName in thread:" + Thread.currentThread() + " is : " + ThreadLocalDemo.threadName.get());
ThreadLocalDemo.threadName.remove();
}
} );
}
System.out.println("ThreadLocalDemo.threadName in main is : " + ThreadLocalDemo.threadName.get());
}
}
输出：
ThreadLocalDemo.threadName in main is : 789
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-1,5,main] is : -1
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-1,5,main] is : 23
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-1,5,main] is : -1
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-2,5,main] is : -1
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-2,5,main] is : 31
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-2,5,main] is : -1
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-2,5,main] is : 93
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-2,5,main] is : -1
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-2,5,main] is : 88
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-2,5,main] is : -1
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-2,5,main] is : 62
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-2,5,main] is : -1
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-2,5,main] is : 53
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-2,5,main] is : -1
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-2,5,main] is : 3
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-2,5,main] is : -1
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-2,5,main] is : 78
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-2,5,main] is : -1
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-2,5,main] is : 48
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-3,5,main] is : -1
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-3,5,main] is : 82
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-3,5,main] is : -1
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-3,5,main] is : 21
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-4,5,main] is : -1
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-4,5,main] is : 91
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-2,5,main] is : -1
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-2,5,main] is : 8
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-5,5,main] is : -1
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-5,5,main] is : 43
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-6,5,main] is : -1
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-7,5,main] is : -1
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-7,5,main] is : 87
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-6,5,main] is : 62
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-8,5,main] is : -1
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-8,5,main] is : 56
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-9,5,main] is : -1
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-9,5,main] is : 76
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-1,5,main] is : 74
Before####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-10,5,main] is : -1
After####ThreadLocalDemo.threadName in thread:Thread[pool-1-thread-10,5,main] is : 47
最常见的ThreadLocal使用场景为 用来解决 数据库连接、Spring的Session管理等。由于框架一般使用多线程机制，即线程复用，所以，每一次进入的控制逻辑的代码时都需要set新的值，并且结束时需要remove。
例如：
private static ThreadLocalconnectionHolder = new ThreadLocal () { public Connection initialValue() { return DriverManager.getConnection(DB_URL); } }; public static Connection getConnection() { return connectionHolder.get(); }
或者：
public static Session getSession() throws InfrastructureException {
Session s = (Session) threadSession.get();
try {
if (s == null) {
s = getSessionFactory().openSession();
threadSession.set(s);
}
} catch (HibernateException ex) {
throw new InfrastructureException(ex);
}
return s;
}