ThreadLocal原理实现

之前有在项目中应用过ThreadLocal的示例，不过用的时候，也只是大致了解了ThreadLocal的应用场景，对于它的实现原理，并没有去深入看过。正巧在办公桌上的《java高并发程序设计》中看到了，遂了解一番，记录一波。
ThreadLocal是线程的局部变量，线程间无法读取彼此的数据，只能在当前线程访问到数据，是线程安全的。常见的应用场景：管理数据库的Connection。

书中的示例代码如下：

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

/**
 * Description: 

 *
 * @author: name：yuxin 
 * Create Time:  2018/5/28 0028-下午 10:10

 */
public class TlTest {
    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;
        }
        public void run(){
            Date t = null;
            try {
                t = sdf.parse("2018-05-28 22:12:"+i%60);
            } catch (ParseException e) {
                e.printStackTrace();
            }
            System.out.println(i+":"+t);
        }
    }

    public static void main(String[] args) {
        ExecutorService es = Executors.newFixedThreadPool(10);
        for (int i = 0; i < 1000; i++) {
            es.execute(new ParseDate(i));
        }
    }
}

由于SimpleDateFormat不是线程安全的，所以在代码运行中报错如下：

Exception in thread "pool-1-thread-18" java.lang.NumberFormatException: For input string: ""
    at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
    at java.lang.Long.parseLong(Long.java:601)
    at java.lang.Long.parseLong(Long.java:631)
    at java.text.DigitList.getLong(DigitList.java:195)
    at java.text.DecimalFormat.parse(DecimalFormat.java:2051)
    at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:2162)
    at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1514)
    at java.text.DateFormat.parse(DateFormat.java:364)
    at TlTest$ParseDate.run(TlTest.java:26)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

使用ThreadLocal解决上述问题

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

/**
 * Description: 

 *
 * @author: name：yuxin
 * Create Time:  2018/5/28 0028-下午 10:10

 */
public class TlTest {
    private static ThreadLocal threadLocal = new ThreadLocal<>();
    public static class ParseDate implements Runnable{
        int i = 0;
        public ParseDate(int i){
            this.i=i;
        }
        public void run(){
            Date t = null;
            if(threadLocal.get()==null){
                threadLocal.set(new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"));
            }
            SimpleDateFormat sdf = threadLocal.get();
            try {
                t = sdf.parse("2018-05-28 22:12:"+i%60);
            } catch (ParseException e) {
                e.printStackTrace();
            }
            System.out.println(i+":"+t);
        }
    }

    public static void main(String[] args) {
        ExecutorService es = Executors.newFixedThreadPool(10);
        for (int i = 0; i < 1000; i++) {
            es.execute(new ParseDate(i));
        }
    }
}

ThreadLocal的实现原理

在上述的应用中，可以了解到ThreadLocal有set(),get()两个方法

set的实现如下

public void set(T value) {
    //1.获取到当前线程对象
    Thread t = Thread.currentThread();
    //2.通过当前线程对象获取到ThreadLocalMap
    ThreadLocalMap map = getMap(t);
    //3.若获取到的ThreadLocalMap不为空，则以当前ThreadLocal对象作为key，将值存储
    if (map != null)
        map.set(this, value);
    else
    //4.若获取到的ThreadLocalMap为空,则创建ThreadLocalMap对象
        createMap(t, value);
}

get的实现如下

public T get() {
    //1.获取到当前线程对象
    Thread t = Thread.currentThread();
    //2.通过当前线程对象获取到ThreadLocalMap
    ThreadLocalMap map = getMap(t);
    //3.若获取到的ThreadLocalMap不为空,则以当前ThreadLocal对象作为key，将值取出,并返回值
    if (map != null) {
        ThreadLocalMap.Entry e = map.getEntry(this);
        if (e != null) {
            @SuppressWarnings("unchecked")
            T result = (T)e.value;
            return result;
        }
    }
    //4.若获取到的ThreadLocalMap为空,则进行初始化，并将初始化的值返回
    return setInitialValue();
}

通过以上代码，我们可以了解到ThreadLocal的值是存在于Thread的内部类ThreadLocalMap中的，在ThreadLocalMap中，key为ThreadLocal当前对象的弱引用。
当线程执行完毕，退出时，为了ThreadLocalMap能尽快被清理，会执行如下操作

    /**
     * This method is called by the system to give a Thread
     * a chance to clean up before it actually exits.
     */
    private void exit() {
        if (group != null) {
            group.threadTerminated(this);
            group = null;
        }
        /* Aggressively null out all reference fields: see bug 4006245 */
        target = null;
        //加速ThreadLocalMap被清理
        threadLocals = null;
        inheritableThreadLocals = null;
        inheritedAccessControlContext = null;
        blocker = null;
        uncaughtExceptionHandler = null;
    }

如上，是在线程结束时，会执行的操作。但是，如果在一个应用了线程池的操作中使用了ThreadLocal，会隐藏着内存泄漏的风险。因为当线程结束后，线程并没有被exit，而是放回了线程池中，以后若不再使用ThreadLocalMap中的值，由于线程一直存在，ThreadLocalMap是无法被回收的。因此在该场景中使用ThreadLocal时，应注意在使用后，及时调用ThreadLocal.remove()方法，该方法代码如下：

/**
 * Removes the current thread's value for this thread-local
 * variable.  If this thread-local variable is subsequently
 * {@linkplain #get read} by the current thread, its value will be
 * reinitialized by invoking its {@link #initialValue} method,
 * unless its value is {@linkplain #set set} by the current thread
 * in the interim.  This may result in multiple invocations of the
 * {@code initialValue} method in the current thread.
 *
 * @since 1.5
 */
 public void remove() {
     ThreadLocalMap m = getMap(Thread.currentThread());
     if (m != null)
         m.remove(this);
 }

ThreadLocalMap中的remove方法代码如下：

/**
 * Remove the entry for key.
 */
private void remove(ThreadLocal key) {
    Entry[] tab = table;
    int len = tab.length;
    int i = key.threadLocalHashCode & (len-1);
    for (Entry e = tab[i];
         e != null;
         e = tab[i = nextIndex(i, len)]) {
        if (e.get() == key) {
            e.clear();
            expungeStaleEntry(i);
            return;
        }
    }
}