美团面试，问了ThreadLocal原理，这个回答让我通过了

Z时代
2024-01-10
分类：技术分享

前言

上周我侥幸通过美团一面（点击查看一面过程），岗位是java后端开发工程师。美团面试官给我进行了二面。面试过程中他问了ThreadLocal原理（上次问线程池，这次问ThreadLocal，美团爸爸这么喜欢线程安全机制么），今天详细讲一讲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

publicvoidset(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
voidcreateMap(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。

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

privatevoidset(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：


privatefinalint threadLocalHashCode = nextHashCode();
/**
* The next hash code to be given out. Updated atomically. Starts at
* zero.
*/
privatestatic 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.
*/
privatestaticfinalint HASH_INCREMENT = 0x61c88647;
/**
* Returns the next hash code.
*/
privatestaticintnextHashCode(){
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.
*/
privatevoidremove(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。

Synchronized是通过线程等待，牺牲时间来解决访问冲突

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;
/**
* <h3>Exper1</h3>
* <p>ThreadLocalId</p>
*
* @author : cxc
* @date : 2020-04-01 23:48
**/
publicclassThreadLocalId{
// Atomic integer containing the next thread ID to be assigned
privatestaticfinal AtomicInteger nextId = new AtomicInteger(0);
// Thread local variable containing each thread's ID
privatestaticfinal ThreadLocal <Integer> threadId =
new ThreadLocal<Integer>()
{
@Override
protected Integer initialValue(){
return nextId.getAndIncrement();
}
};
// Returns the current thread's unique ID, assigning it if necessary
publicstaticintget(){
return threadId.get();
}
publicstaticvoidremove(){
threadId.remove();
}
}
/**
* <h3>Exper1</h3>
* <p></p>
*
* @author : cxc
* @date : 2020-04-02 00:07
**/
publicclassThreadLocalMain{
privatestaticvoidincrementSameThreadId(){
try{
for(int i=0;i<5;i++){
System.out.println(Thread.currentThread()
+"_"+i+",threadId:"+
ThreadLocalId.get());
}
}finally {
ThreadLocalId.remove();
}
}
publicstaticvoidmain(String[] args){
incrementSameThreadId();
new Thread(new Runnable() {
@Override
publicvoidrun(){
incrementSameThreadId();
}
}).start();
new Thread(new Runnable() {
@Override
publicvoidrun(){
incrementSameThreadId();
}
}).start();
}
}