美团面试，问了ThreadLocal原理，这个回答让我通过了
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。
ThreadLocalThreadLocal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县城有不同数据副本时，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。
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;
int i = key.threadLocalHashCode & (len-1);
for (Entry e = tab[i];
e != null;
e = tab[i = nextIndex(i, len)]) {
ThreadLocal k = e.get();
if (k == key) {
e.value = value;
return;
}
if (k == null) {
replaceStaleEntry(key, value, i);
return;
}
}
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。
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;
/**
* 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 threadId =
new ThreadLocal()
{
@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。
作者：天才程序YUAN
