美团Java后端面试题,巧妙的回答ThreadLocal原理!

想要去好点的公司,想要去前景好的公司都对技术要求挺高的,面试时技术问也会相应的难些,就拿美团来说,它好像比较喜欢线程安全机制问题,之前就有小伙伴被问倒了!所以今天就详细讲一讲ThreadLocal原理。

ThreadLocal
ThreadLocal是线程的内部存储类,可以在指定线程内存储数据。只有指定线程可以得到存储数据。

/**
 * 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).
 */

每个线程都有一个ThreadLocalMap的实例对象,并且通过ThreadLocal管理ThreadLocalMap。

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

每个新线程都会实例化为一个ThreadLocalMap并且赋值给成员变量ThreadLocals,使用时若已经存在threadLocals则直接使用已经存在的对象。

应用场景
当某些数据是以线程为作用域并且不同线程有不同数据副本时,考虑ThreadLocal。

无状态,副本变量独立后不影响业务逻辑的高并发场景。

如果如果业务逻辑强依赖于副本变量,则不适合用ThreadLocal解决。
get()与set()
set()是调用ThreadLocalMap的set()实现的:

public void set(T value) {
     
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }
//getMap方法
ThreadLocalMap getMap(Thread t) {
     
      //thred中维护了一个ThreadLocalMap
      return t.threadLocals;
 }

//createMap
void createMap(Thread t, T firstValue) {
     
      //实例化一个新的ThreadLocalMap,并赋值给线程的成员变量threadLocals
      t.threadLocals = new ThreadLocalMap(this, firstValue);
}

ThreadLocalMap:
ThreadLocalMap为每个Thread都维护了一个数组table,ThreadLocal确定了一个数组下标,而这个下标是value存储的对应位置。

[图片上传中…(image-cd716a-1587459684812-0)]

ThreadLocalMaps是延迟构造的,因此只有在至少要放置一个条目时才创建。

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);
        }

ThreadLocalMap初始化时创建了默认长度是16的Entry数组。通过hashCode与length位运算确定索引值i。

每个Thread都有一个ThreadLocalMap类型。相当于每个线程Thread都有一个Entry型的数组table。而一切读取过程都是通过操作这个数组table进行的。

set() 方法

    private void set(ThreadLocal<?> key, Object value) {
     
            // We don't use a fast path as with get() because it is at
            // least as common to use set() to create new entries as
            // it is to replace existing ones, in which case, a fast
            // path would fail more often than not.

            Entry[] tab = table;
            int len = tab.length;
            //通过&运算计算索引
            int i = key.threadLocalHashCode & (len-1);

            for (Entry e = tab[i];
                 e != null;
                 e = tab[i = nextIndex(i, len)]) {
     
                ThreadLocal<?> k = e.get();
                //如果存在key则覆盖
                if (k == key) {
     
                    e.value = value;
                    return;
                }
                if (k == null) {
     
                    replaceStaleEntry(key, value, i);
                    return;
                }
            }
            //新建结点插入
            tab[i] = new Entry(key, value);
            int sz = ++size;
            if (!cleanSomeSlots(i, sz) && sz >= threshold)
                rehash();
        }

将threadLocalHashCode与长度进行位运算得到索引。

threadLocalHashCode的代码如下:

    private final int threadLocalHashCode = nextHashCode();
   /**
     * The next hash code to be given out. Updated atomically. Starts at
     * zero.
     */
    private static AtomicInteger nextHashCode =
        new AtomicInteger();

    /**
     * The difference between successively generated hash codes - turns
     * implicit sequential thread-local IDs into near-optimally spread
     * multiplicative hash values for power-of-two-sized tables.
     */
    private static final int HASH_INCREMENT = 0x61c88647;

    /**
     * Returns the next hash code.
     */
    private static int nextHashCode() {
     
        return nextHashCode.getAndAdd(HASH_INCREMENT);
    }

由于是static变量,threadLocalHashCode在每次加载threadLocal类时会重新初始化,同时会自增一次,增加HASH_INCREMENT(斐波那契散列乘数,通过该数散列出来的结果会比较均匀)。

static变量也称作静态变量,静态变量和非静态变量的区别是:静态变量被所有的对象所共享,在内存中只有一个副本,它当且仅当在类初次加载时会被初始化。

而非静态变量是对象所拥有的,在创建对象的时候被初始化,存在多个副本,各个对象拥有的副本互不影响。static成员变量的初始化顺序按照定义的顺序进行初始化。

对于一个ThreadLocal来讲,他的索引值i是确定的。对于不同线程,同一个threadlocal对应的是不同table的同一下标,即是table[i],不同线程之间的table是相互独立的。

get() 方法
计算索引,直接取出:

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();
    }

remove() 方法

/**
  * 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;
           }
       }
   }

线程隔离特性:
线程隔离特性,只有在线程内才能获取到对应的值,线程外不能访问。

(1)Synchronized是通过线程等待,牺牲时间来解决访问冲突。
(2)ThreadLocal是通过每个线程单独一份存储空间,牺牲空间来解决冲突。

内存泄露问题:
存在内存泄露问题,每次使用完ThreadLocal,都调用它的remove()方法,清除数据。

Demo程序:

import java.util.concurrent.atomic.AtomicInteger;
/**
 * 

Exper1

*

ThreadLocalId

* * @author : cxc * @date : 2020-04-01 23:48 **/
public class ThreadLocalId { // Atomic integer containing the next thread ID to be assigned private static final AtomicInteger nextId = new AtomicInteger(0); // Thread local variable containing each thread's ID private static final ThreadLocal <Integer> threadId = new ThreadLocal<Integer>() { @Override protected Integer initialValue() { return nextId.getAndIncrement(); } }; // Returns the current thread's unique ID, assigning it if necessary public static int get() { return threadId.get(); } public static void remove() { threadId.remove(); } } /** *

Exper1

*

* * @author : cxc * @date : 2020-04-02 00:07 **/
public class ThreadLocalMain { private static void incrementSameThreadId(){ try{ for(int i=0;i<5;i++){ System.out.println(Thread.currentThread() +"_"+i+",threadId:"+ ThreadLocalId.get()); } }finally { ThreadLocalId.remove(); } } public static void main(String[] args) { incrementSameThreadId(); new Thread(new Runnable() { @Override public void run() { incrementSameThreadId(); } }).start(); new Thread(new Runnable() { @Override public void run() { incrementSameThreadId(); } }).start(); } }

总结:

咱们玩归玩,闹归闹,别拿面试开玩笑。ThreadLocal的原理在面试中几乎被问烂了。Thread的私有数据是存储在ThreadLocalMap,通过ThreadLoacl进行管理。要了解ThreadLocal的原理,最好多阅读几遍源码,尤其是ThreadLocalMap的源码部分。大家面试前要把知识点记牢。要是需要更多的Java后端的面试资料或者线程方面学习资料的可以点击这里,暗号:cszq,也可以关注+私信我,免费提供!
美团Java后端面试题,巧妙的回答ThreadLocal原理!_第1张图片
美团Java后端面试题,巧妙的回答ThreadLocal原理!_第2张图片
最后祝大家工作都能顺顺利利哦!

你可能感兴趣的:(Java面试,Java后端,线程安全,java,后端,thread,安全)