Java 中的本地线程 ThreadLocal 与同步机制的比较和最佳实践

是什么?

java.lang.ThreadLocal extends Object

ThreadLocal 类提供线程局部变量。

ThreadLocal 解决线程安全持有对象访问的问题 . 通过 ThreadLocal.set() 方法将对象实例保存在每个线程自己所拥有的 ThreadLocalMap中，这样每个线程使用自己的对象实例，彼此不会影响达到隔离的作用，从而就解决了对象在被共享访问带来线程安全问题。

如果将同步机制和 ThreadLocal 做一个横向比较的话，同步机制就是通过控制线程访问共享对象的顺序，而 ThreadLocal 就是为每一个线程分配一个该对象，各用各的互不影响。

打个比方说，现在有100个同学需要填写一张表格但是只有一支笔，同步就相当于A使用完这支笔后给B，B使用后给C用......老师就控制着这支笔的使用顺序，使得同学之间不会产生冲突。而 ThreadLocal 就相当于，老师直接准备了100支笔，这样每个同学都使用自己的，同学之间就不会产生冲突。

很显然这就是两种不同的思路，同步机制以“时间换空间”，由于每个线程在同一时刻共享对象只能被一个线程访问造成整体上响应时间增加，但是对象只占有一份内存，牺牲了时间效率换来了空间效率即“时间换空间”。而ThreadLocal，为每个线程都分配了一份对象，自然而然内存使用率增加，每个线程各用各的，整体上时间效率要增加很多，牺牲了空间效率换来时间效率即“空间换时间”。

应用场景

org.apache.ibatis.session.SqlSessionManager

/**
 *    Copyright 2009-2018 the original author or authors.
 *
 *    Licensed under the Apache License, Version 2.0 (the "License");
 *    you may not use this file except in compliance with the License.
 *    You may obtain a copy of the License at
 *
 *       http://www.apache.org/licenses/LICENSE-2.0
 *
 *    Unless required by applicable law or agreed to in writing, software
 *    distributed under the License is distributed on an "AS IS" BASIS,
 *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *    See the License for the specific language governing permissions and
 *    limitations under the License.
 */
package org.apache.ibatis.session;

import java.io.InputStream;
import java.io.Reader;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.sql.Connection;
import java.util.List;
import java.util.Map;
import java.util.Properties;

import org.apache.ibatis.cursor.Cursor;
import org.apache.ibatis.executor.BatchResult;
import org.apache.ibatis.reflection.ExceptionUtil;

/**
 * @author Larry Meadors
 */
public class SqlSessionManager implements SqlSessionFactory, SqlSession {

  private final SqlSessionFactory sqlSessionFactory;
  private final SqlSession sqlSessionProxy;

  private final ThreadLocal localSqlSession = new ThreadLocal<>();
...

  public void startManagedSession() {
    this.localSqlSession.set(openSession());
  }

  public void startManagedSession(boolean autoCommit) {
    this.localSqlSession.set(openSession(autoCommit));
  }

  public void startManagedSession(Connection connection) {
    this.localSqlSession.set(openSession(connection));
  }

  public void startManagedSession(TransactionIsolationLevel level) {
    this.localSqlSession.set(openSession(level));
  }

  public void startManagedSession(ExecutorType execType) {
    this.localSqlSession.set(openSession(execType));
  }

  public void startManagedSession(ExecutorType execType, boolean autoCommit) {
    this.localSqlSession.set(openSession(execType, autoCommit));
  }

  public void startManagedSession(ExecutorType execType, TransactionIsolationLevel level) {
    this.localSqlSession.set(openSession(execType, level));
  }

  public void startManagedSession(ExecutorType execType, Connection connection) {
    this.localSqlSession.set(openSession(execType, connection));
  }

  public boolean isManagedSessionStarted() {
    return this.localSqlSession.get() != null;
  }

  ...

  @Override
  public Connection getConnection() {
    final SqlSession sqlSession = localSqlSession.get();
    if (sqlSession == null) {
      throw new SqlSessionException("Error:  Cannot get connection.  No managed session is started.");
    }
    return sqlSession.getConnection();
  }

  @Override
  public void clearCache() {
    final SqlSession sqlSession = localSqlSession.get();
    if (sqlSession == null) {
      throw new SqlSessionException("Error:  Cannot clear the cache.  No managed session is started.");
    }
    sqlSession.clearCache();
  }

  @Override
  public void commit() {
    final SqlSession sqlSession = localSqlSession.get();
    if (sqlSession == null) {
      throw new SqlSessionException("Error:  Cannot commit.  No managed session is started.");
    }
    sqlSession.commit();
  }

  @Override
  public void commit(boolean force) {
    final SqlSession sqlSession = localSqlSession.get();
    if (sqlSession == null) {
      throw new SqlSessionException("Error:  Cannot commit.  No managed session is started.");
    }
    sqlSession.commit(force);
  }

  @Override
  public void rollback() {
    final SqlSession sqlSession = localSqlSession.get();
    if (sqlSession == null) {
      throw new SqlSessionException("Error:  Cannot rollback.  No managed session is started.");
    }
    sqlSession.rollback();
  }

  @Override
  public void rollback(boolean force) {
    final SqlSession sqlSession = localSqlSession.get();
    if (sqlSession == null) {
      throw new SqlSessionException("Error:  Cannot rollback.  No managed session is started.");
    }
    sqlSession.rollback(force);
  }

  @Override
  public List flushStatements() {
    final SqlSession sqlSession = localSqlSession.get();
    if (sqlSession == null) {
      throw new SqlSessionException("Error:  Cannot rollback.  No managed session is started.");
    }
    return sqlSession.flushStatements();
  }

  @Override
  public void close() {
    final SqlSession sqlSession = localSqlSession.get();
    if (sqlSession == null) {
      throw new SqlSessionException("Error:  Cannot close.  No managed session is started.");
    }
    try {
      sqlSession.close();
    } finally {
      localSqlSession.set(null);
    }
  }

  private class SqlSessionInterceptor implements InvocationHandler {
    public SqlSessionInterceptor() {
        // Prevent Synthetic Access
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
      final SqlSession sqlSession = SqlSessionManager.this.localSqlSession.get();
      if (sqlSession != null) {
        try {
          return method.invoke(sqlSession, args);
        } catch (Throwable t) {
          throw ExceptionUtil.unwrapThrowable(t);
        }
      } else {
        try (SqlSession autoSqlSession = openSession()) {
          try {
            final Object result = method.invoke(autoSqlSession, args);
            autoSqlSession.commit();
            return result;
          } catch (Throwable t) {
            autoSqlSession.rollback();
            throw ExceptionUtil.unwrapThrowable(t);
          }
        }
      }
    }
  }

}

实现原理: ThreadLocal的设计

首先看看ThreadLocal的接口：

protected T initialValue(); // 返回该线程局部变量的当前线程的初始值                   
public void set(T value); // 设置当前线程的线程局部变量副本的值
public T get();  // 返回当前线程的线程局部变量副本 

ThreadLocal有3个方法，其中值得注意的是initialValue()，该方法是一个protected 的方法，显然是为了子类重写而特意实现的。该方法返回当前线程在该线程局部变量的初始值，这个方法是一个延迟调用方法，在一个线程第1次调用get()或者set(Object)时才执行，并且仅执行1次。ThreadLocal中的确实实现直接返回一个null：

    /**
     * Returns the current thread's "initial value" for this
     * thread-local variable.  This method will be invoked the first
     * time a thread accesses the variable with the {@link #get}
     * method, unless the thread previously invoked the {@link #set}
     * method, in which case the {@code initialValue} method will not
     * be invoked for the thread.  Normally, this method is invoked at
     * most once per thread, but it may be invoked again in case of
     * subsequent invocations of {@link #remove} followed by {@link #get}.
     *
     * 
This implementation simply returns {@code null}; if the
     * programmer desires thread-local variables to have an initial
     * value other than {@code null}, {@code ThreadLocal} must be
     * subclassed, and this method overridden.  Typically, an
     * anonymous inner class will be used.
     *
     * @return the initial value for this thread-local
     */
    protected T initialValue() {
        return null;
    }

ThreadLocalMap

ThreadLocal是如何做到为每一个线程维护变量的副本的呢？其实实现的思路很简单，在ThreadLocal类中有一个Map，用于存储每一个线程的变量的副本。

为什么不直接使用Map为底层数据结构?

如果ThreadLoad直接使用Map为底层数据结构，当有大量的线程使用ThreadLocal时，首先Map访问的性能会下降，伴随着线程生命周期，底层的Map还需要频繁的添加删除entity，这就很容易造成性能瓶颈。

ThreadLocalMap 源码:

    /**
     * ThreadLocalMap is a customized hash map suitable only for
     * maintaining thread local values. No operations are exported
     * outside of the ThreadLocal class. The class is package private to
     * allow declaration of fields in class Thread.  To help deal with
     * very large and long-lived usages, the hash table entries use
     * WeakReferences for keys. However, since reference queues are not
     * used, stale entries are guaranteed to be removed only when
     * the table starts running out of space.
     */
    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 [清除] from table.  Such entries are referred to
         * as "stale【陈腐的；不新鲜的】 entries" in the code that follows.
         */
        static class Entry extends WeakReference> {
            /** 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);
        }

        /**
         * Construct a new map including all Inheritable ThreadLocals
         * from given parent map. Called only by createInheritedMap.
         *
         * @param parentMap the map associated with parent thread.
         */
        private ThreadLocalMap(ThreadLocalMap parentMap) {
            Entry[] parentTable = parentMap.table;
            int len = parentTable.length;
            setThreshold(len);
            table = new Entry[len];

            for (int j = 0; j < len; j++) {
                Entry e = parentTable[j];
                if (e != null) {
                    @SuppressWarnings("unchecked")
                    ThreadLocal