ThreadLocal

1. 什么是ThreadLocal

我们先来看下JDK 的文档介绍

/**
 * This class provides thread-local variables.  These variables differ from
 * their normal counterparts in that each thread that accesses one (via its
 * {@code get} or {@code set} method) has its own, independently initialized
 * copy of the variable.  {@code ThreadLocal} instances are typically private
 * static fields in classes that wish to associate state with a thread (e.g.,
 * a user ID or Transaction ID).
 * 
 * <p>For example, the class below generates unique identifiers local to each
 * thread.
 * A thread's id is assigned the first time it invokes {@code ThreadId.get()}
 * and remains unchanged on subsequent calls.
 */

ThreadLocal提供了线程的局部变量。每个线程都可以通过set()get()来对这个 局部变量进行操作,但不会和其他线程的局部变量进行冲突。实现了线程的数据隔离

简要言之:往ThreadLocal中填充的变量属于当前线程,该变量对其他线程而言是隔离的。

2. 为什么要学习ThreadLocal

从上面可以得出:ThreadLocal可以让我们拥有当前线程的变量,那这个作用有什么用呢???

2.1 管理Connection

最典型的是管理数据库的Connection

当时在学JDBC的时候,为了方便操作写了一个简单数据库连接池,需要数据库连接池的理由也很简单,频繁创建和关闭Connection是一件非常耗费资源的操作,因此需要创建数据库连接池~

那么,数据库连接池的连接怎么管理呢??我们交由ThreadLocal来进行管理。为什么交给它来管理呢??ThreadLocal能够实现当前线程的操作都是用同一个Connection,保证了事务!

当时候写的代码:

public class DBUtil {
    //数据库连接池
    private static BasicDataSource source;

    //为不同的线程管理连接
    private static ThreadLocal<Connection> local;


    static {
        try {
            //加载配置文件
            Properties properties = new Properties();

            //获取读取流
            InputStream stream = DBUtil.class.getClassLoader().getResourceAsStream("连接池/config.properties");

            //从配置文件中读取数据
            properties.load(stream);

            //关闭流
            stream.close();

            //初始化连接池
            source = new BasicDataSource();

            //设置驱动
            source.setDriverClassName(properties.getProperty("driver"));

            //设置url
            source.setUrl(properties.getProperty("url"));

            //设置用户名
            source.setUsername(properties.getProperty("user"));

            //设置密码
            source.setPassword(properties.getProperty("pwd"));

            //设置初始连接数量
            source.setInitialSize(Integer.parseInt(properties.getProperty("initsize")));

            //设置最大的连接数量
            source.setMaxActive(Integer.parseInt(properties.getProperty("maxactive")));

            //设置最长的等待时间
            source.setMaxWait(Integer.parseInt(properties.getProperty("maxwait")));

            //设置最小空闲数
            source.setMinIdle(Integer.parseInt(properties.getProperty("minidle")));

            //初始化线程本地
            local = new ThreadLocal<>();


        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static Connection getConnection() throws SQLException {

        if(local.get()!=null){
            return local.get();
        }else{

            //获取Connection对象
            Connection connection = source.getConnection();

            //把Connection放进ThreadLocal里面
            local.set(connection);

            //返回Connection对象
            return connection;
        }

    }

    //关闭数据库连接
    public static void closeConnection() {
        //从线程中拿到Connection对象
        Connection connection = local.get();

        try {
            if (connection != null) {
                //恢复连接为自动提交
                connection.setAutoCommit(true);

                //这里不是真的把连接关了,只是将该连接归还给连接池
                connection.close();

                //既然连接已经归还给连接池了,ThreadLocal保存的Connction对象也已经没用了
                local.remove();

            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }


}

同样的,Hibernate对Connection的管理也是采用了相同的手法(使用ThreadLocal,当然了Hibernate的实现是更强大的)~

2.2 避免一些参数传递

避免一些参数的传递的理解可以参考一下Cookie和Session:

3. ThreadLocal 实现的原理

 public void set(T value) {

        // 得到当前线程对象
        Thread t = Thread.currentThread();

        // 这里获取ThreadLocalMap
        ThreadLocalMap map = getMap(t);

        // 如果map存在,则将当前线程对象t作为key,要存储的对象作为value存到map里面去
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }

上面有个ThreadLocalMap,我们去看看这是什么?

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;
            }
        }
        //....很长
}

通过上面我们可以发现的是ThreadLocalMap是ThreadLocal的一个内部类。用Entry类来进行存储

我们的值都是存储到这个Map上的,key是当前ThreadLocal对象

如果该Map不存在,则初始化一个:

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

如果该Map存在,则从Thread中获取

  /**
     * Get the map associated with a ThreadLocal. Overridden in
     * InheritableThreadLocal.
     *
     * @param  t the current thread
     * @return the map
     */
    ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }

Thread维护了ThreadLocalMap变量

  /* ThreadLocal values pertaining to this thread. This map is maintained
     * by the ThreadLocal class. */
    ThreadLocal.ThreadLocalMap threadLocals = null

从上面又可以看出,ThreadLocalMap是在ThreadLocal中使用内部类来编写的,但对象的引用是在Thread中

于是我们可以总结出:Thread为每个线程维护了ThreadLocalMap这么一个Map,而ThreadLocalMap的key是LocalThread对象本身,value则是要存储的对象

有了上面的基础,我们看get()方法就一点都不难理解了

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();
    }

3.1 ThreadLocal 原理总结

  1. 每个Thread维护着一个ThreadLocalMap的引用
  2. ThreadLocalMap 是ThreadLocal的内部类,用Entry来进行存储
  3. 调用ThreadLocal的set()方法时,实际上就是往ThreadLocalMap设置值,key是ThreadLocal对象,值是传递进来的对象
  4. 调用ThreadLocal的get()方法时,实际上就是往ThreadLocalMap获取值,key是ThreadLocal对象
  5. ThreadLocal本身并不存储值,它只是作为一个key来让线程从ThreadLocalMap获取value

4. 避免内存泄漏

我们来看一下ThreadLocal的对象关系引用图:

image-20190917002342817

ThreadLocal内存泄漏的根源是:由于ThreadLocalMap的生命周期跟Thread一样长,如果没有手动删除对应key就会导致内存泄漏,而不是因为弱引用

想要避免内存泄露就要手动remove()掉

5. 总结

ThreadLocal设计的目的就是为了能够在当前线程中有属于自己的变量,并不是为了解决并发或者共享变量的问题

参考文章

ThreadLocal就是这么简单

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