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Java多线程 ThreadLocal原理解析

2022-03-05 15:04冬日毛毛雨 Java教程

这篇文章主要介绍了Java多线程 ThreadLocal原理,ThreadLoal 变量,线程局部变量,同一个 ThreadLocal 所包含的对象,在不同的 Thread 中有不同的副本,下面文章也是围绕Java多线程 ThreadLocal展开内容,需要的朋友可以参考一下

1、什么是ThreadLocal变量

ThreadLoal 变量,线程局部变量,同一个 ThreadLocal 所包含的对象,在不同的 Thread 中有不同的副本。

这里有几点需要注意:

  • 因为每个 Thread 内有自己的实例副本,且该副本只能由当前 Thread 使用。这是也是 ThreadLocal 命名的由来。
  • 既然每个 Thread 有自己的实例副本,且其它 Thread 不可访问,那就不存在多线程间共享的问题。

ThreadLocal 提供了线程本地的实例。它与普通变量的区别在于,每个使用该变量的线程都会初始化一个完全独立的实例副本。ThreadLocal 变量通常被private static修饰。当一个线程结束时,它所使用的所有 ThreadLocal 相对的实例副本都可被回收。

总的来说,ThreadLocal 适用于每个线程需要自己独立的实例且该实例需要在多个方法中被使用,也即变量在线程间隔离而在方法或类间共享的场景。

2、ThreadLocal实现原理

首先 ThreadLocal 是一个泛型类,保证可以接受任何类型的对象。

因为一个线程内可以存在多个 ThreadLocal 对象,所以其实是 ThreadLocal 内部维护了一个 Map ,这个 Map 不是直接使用的 HashMap ,而是 ThreadLocal 实现的一个叫做 ThreadLocalMap 的静态内部类。而我们使用的 get()set() 方法其实都是调用了这个ThreadLocalMap类对应的 get() set() 方法。

例如下面的 set 方法:

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public void set(T value) {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null)
        map.set(this, value);
    else
        createMap(t, value);
}

get方法:

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public T get() {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null) {
        ThreadLocalMap.Entry e = map.getEntry(this);
        if (e != null) {
            @SuppressWarnings("unchecked")
            T result = (T)e.value;
            return result;
        }
    }
    return setInitialValue();
}

createMap方法:

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void createMap(Thread t, T firstValue) {
    t.threadLocals = new ThreadLocalMap(this, firstValue);
}

ThreadLocalMap是个静态的内部类:

 

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static class ThreadLocalMap {
 
        /**
         * The entries in this hash map extend WeakReference, using
         * its main ref field as the key (which is always a
         * ThreadLocal object).  Note that null keys (i.e. entry.get()
         * == null) mean that the key is no longer referenced, so the
         * entry can be expunged from table.  Such entries are referred to
         * as "stale entries" in the code that follows.
         */
        static class Entry extends WeakReference<ThreadLocal<?>> {
            /** The value associated with this ThreadLocal. */
            Object value;
 
            Entry(ThreadLocal<?> k, Object v) {
                super(k);
                value = v;
            }
        }
 
        /**
         * The initial capacity -- MUST be a power of two.
         */
        private static final int INITIAL_CAPACITY = 16;
 
        /**
         * The table, resized as necessary.
         * table.length MUST always be a power of two.
         */
        private Entry[] table;
 
        /**
         * The number of entries in the table.
         */
        private int size = 0;
 
        /**
         * The next size value at which to resize.
         */
        private int threshold; // Default to 0
 
        /**
         * Set the resize threshold to maintain at worst a 2/3 load factor.
         */
        private void setThreshold(int len) {
            threshold = len * 2 / 3;
        }
 
        /**
         * Increment i modulo len.
         */
        private static int nextIndex(int i, int len) {
            return ((i + 1 < len) ? i + 1 : 0);
        }
 
        /**
         * Decrement i modulo len.
         */
        private static int prevIndex(int i, int len) {
            return ((i - 1 >= 0) ? i - 1 : len - 1);
        }
 
        /**
         * Construct a new map initially containing (firstKey, firstValue).
         * ThreadLocalMaps are constructed lazily, so we only create
         * one when we have at least one entry to put in it.
         */
        ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
            table = new Entry[INITIAL_CAPACITY];
            int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
            table[i] = new Entry(firstKey, firstValue);
            size = 1;
            setThreshold(INITIAL_CAPACITY);
        }
...
}

最终的变量是放在了当前线程的 ThreadLocalMap 中,并不是存在 ThreadLocal 上,ThreadLocal 可以理解为只是ThreadLocalMap的封装,传递了变量值。

3、内存泄漏问题

实际上 ThreadLocalMap 中使用的 key 为 ThreadLocal 的弱引用,弱引用的特点是,如果这个对象只存在弱引用,那么在下一次垃圾回收的时候必然会被清理掉。

所以如果 ThreadLocal 没有被外部强引用的情况下,在垃圾回收的时候会被清理掉的,这样一来 ThreadLocalMap中使用这个 ThreadLocal 的 key 也会被清理掉。但是,value 是强引用,不会被清理,这样一来就会出现 key 为 null 的 value。

ThreadLocalMap实现中已经考虑了这种情况,在调用 set()get() remove() 方法的时候,会清理掉 key 为 null 的记录。如果说会出现内存泄漏,那只有在出现了 key 为 null 的记录后,没有手动调用 remove() 方法,并且之后也不再调用 get() set()remove() 方法的情况下。

4、使用场景

如上文所述,ThreadLocal 适用于如下两种场景

每个线程需要有自己单独的实例
实例需要在多个方法中共享,但不希望被多线程共享
对于第一点,每个线程拥有自己实例,实现它的方式很多。例如可以在线程内部构建一个单独的实例。ThreadLoca 可以以非常方便的形式满足该需求。

对于第二点,可以在满足第一点(每个线程有自己的实例)的条件下,通过方法间引用传递的形式实现。ThreadLocal 使得代码耦合度更低,且实现更优雅。

1)存储用户Session

一个简单的用ThreadLocal来存储Session的例子:

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private static final ThreadLocal threadSession = new ThreadLocal();
 
    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;
    }

2)解决线程安全的问题

比如Java7中的SimpleDateFormat不是线程安全的,可以用ThreadLocal来解决这个问题:

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public class DateUtil {
    private static ThreadLocal<SimpleDateFormat> format1 = new ThreadLocal<SimpleDateFormat>() {
        @Override
        protected SimpleDateFormat initialValue() {
            return new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        }
    };
 
    public static String formatDate(Date date) {
        return format1.get().format(date);
    }
}

这里的DateUtil.formatDate()就是线程安全的了。(Java8里的 [java.time.format.DateTimeFormatter]

(http://docs.oracle.com/javase/8/docs/api/java/time/format/DateTimeFormatter.html) 是线程安全的,Joda time里的DateTimeFormat也是线程安全的)。

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public class Context {
 
    private String name;
    private String cardId;
 
    public String getCardId() {
        return cardId;
    }
 
    public void setCardId(String cardId) {
        this.cardId = cardId;
    }
 
    public String getName() {
        return this.name;
    }
 
    public void setName(String name) {
        this.name = name;
    }
}
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public class ExecutionTask implements Runnable {
 
    private QueryFromDBAction queryAction = new QueryFromDBAction();
 
    private QueryFromHttpAction httpAction = new QueryFromHttpAction();
 
    @Override
    public void run() {
 
        final Context context = new Context();
        queryAction.execute(context);
        System.out.println("The name query successful");
        httpAction.execute(context);
        System.out.println("The cardId query successful");
 
        System.out.println("The Name is " + context.getName() + " and CardId " + context.getCardId());
    }
}
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public class QueryFromDBAction {
 
    public void execute(Context context) {
 
        try {
            Thread.sleep(1000L);
            String name = "Jack " + Thread.currentThread().getName();
            context.setName(name);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}
 
 
    public void execute(Context context) {
        String name = context.getName();
        String cardId = getCardId(name);
        context.setCardId(cardId);
    }
 
    private String getCardId(String name) {
        try {
            Thread.sleep(1000L);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return "444555" + Thread.currentThread().getId();
    }
}
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public class ContextTest {
 
    public static void main(String[] args) {
 
        IntStream.range(1, 5)
                .forEach(i ->
                        new Thread(new ExecutionTask()).start()
                );
    }
}

The name query successful
The name query successful
The name query successful
The name query successful
The cardId query successful
The Name is Jack Thread-0 and CardId 44455511
The cardId query successful
The Name is Jack Thread-1 and CardId 44455512
The cardId query successful
The Name is Jack Thread-2 and CardId 44455513
The cardId query successful
The Name is Jack Thread-3 and CardId 44455514

问题:需要在每个调用Context的方法中传入进去

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public void execute(Context context) {
}

3)使用ThreadLocal重新设计一个上下文设计模式

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public final class ActionContext {
 
    private static final ThreadLocal<Context> threadLocal = new ThreadLocal() {
        @Override
        protected Object initialValue() {
            return new Context();
        }
    };
 
    public static ActionContext getActionContext() {
        return ContextHolder.actionContext;
    }
 
    public Context getContext() {
 
        return threadLocal.get();
    }
 
    private static class ContextHolder {
        private final static ActionContext actionContext = new ActionContext();
 
    }
}
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public class Context {
 
    private String name;
    private String cardId;
 
    public String getCardId() {
        return cardId;
    }
 
    public void setCardId(String cardId) {
        this.cardId = cardId;
    }
 
    public String getName() {
        return this.name;
    }
 
    public void setName(String name) {
        this.name = name;
    }
}
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public class ExecutionTask implements Runnable {
 
    private QueryFromDBAction queryAction = new QueryFromDBAction();
 
    private QueryFromHttpAction httpAction = new QueryFromHttpAction();
 
    @Override
    public void run() {
 
        queryAction.execute();
        System.out.println("The name query successful");
        httpAction.execute();
        System.out.println("The cardId query successful");
 
        final Context context = ActionContext.getActionContext().getContext();
        System.out.println("The Name is " + context.getName() + " and CardId " + context.getCardId());
    }
}
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public class QueryFromDBAction {
 
    public void execute() {
 
        try {
            Thread.sleep(1000L);
            String name = "Jack " + Thread.currentThread().getName();
            ActionContext.getActionContext().getContext().setName(name);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

public class QueryFromHttpAction {

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    public void execute() {
        Context context = ActionContext.getActionContext().getContext();
        String name = context.getName();
        String cardId = getCardId(name);
        context.setCardId(cardId);
 
 
    }
 
    private String getCardId(String name) {
        try {
            Thread.sleep(1000L);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return "444555" + Thread.currentThread().getId();
    }
}
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public class ContextTest {
 
    public static void main(String[] args) {
 
        IntStream.range(1, 5)
                .forEach(i ->
                        new Thread(new ExecutionTask()).start()
                );
    }
}

The name query successful
The name query successful
The name query successful
The name query successful
The cardId query successful
The Name is Jack Thread-3 and CardId 44455514
The cardId query successful
The cardId query successful
The Name is Jack Thread-0 and CardId 44455511
The cardId query successful
The Name is Jack Thread-2 and CardId 44455513
The Name is Jack Thread-1 and CardId 44455512

这样写 执行过程中不会看到context的定义和声明

注意:在使用之前记得将上个线程中context旧值清除调,否则会重复调用(比如线程池操作)

4)ThreadLocal注意事项

脏数据

线程复用会产生脏数据。由于结程池会重用Thread对象,那么与Thread绑定的类的静态属性ThreadLocal变量也会被重用。如果在实现的线程run()方法体中不显式地调用remove() 清理与线程相关的ThreadLocal信息,那么倘若下一个结程不调用set() 设置初始值,就可能get() 到重用的线程信息,包括 ThreadLocal所关联的线程对象的value值。

内存泄漏

通常我们会使用使用static关键字来修饰ThreadLocal(这也是在源码注释中所推荐的)。在此场景下,其生命周期就不会随着线程结束而结束,寄希望于ThreadLocal对象失去引用后,触发弱引用机制来回收EntryValue就不现实了。如果不进行remove() 操作,那么这个线程执行完成后,通过ThreadLocal对象持有的对象是不会被释放的。

以上两个问题的解决办法很简单,就是在每次用完ThreadLocal时, 必须要及时调用 remove()方法清理。

父子线程共享线程变量

很多场景下通过ThreadLocal来透传全局上下文,会发现子线程的value和主线程不一致。比如用ThreadLocal来存储监控系统的某个标记位,暂且命名为traceId。某次请求下所有的traceld都是一致的,以获得可以统一解析的日志文件。但在实际开发过程中,发现子线程里的traceld为null,跟主线程的并不一致。这就需要使用InheritableThreadLocal来解决父子线程之间共享线程变量的问题,使整个连接过程中的traceId一致。

到此这篇关于Java多线程 ThreadLocal原理解析的文章就介绍到这了,更多相关Java多线程 ThreadLocal内容请搜索服务器之家以前的文章或继续浏览下面的相关文章希望大家以后多多支持服务器之家!

原文链接:https://juejin.cn/post/7022879808967639070

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