说明：Thread中有threadLocals成员变量，threadLocal会在threadlocal首次set时进行赋值【这会在非main线程中复现，主线程启动即会进行赋值】，ThreadLocalMap是ThreadLocal的静态内部类，在set时，会将我们新建threadLocal引用地址作为key，以此封装成一个Entry<threadLocal<?>,Object>对象，可以存在多个不同的threadlocal,如果set的引用地址相同，就会进行覆盖，此处key的类型ThreadLocal继承弱引用也是会造成内存泄露的主要原因，在下面源码中会对此段相关点分别说明。
2.源码解析package com.adun.test_threadlocal;
/**
* @author ADun
* @date 2022/4/27 11:14
*/
public class ThreadLocalTest {
public static final ThreadLocal<Integer> threadLocal = new ThreadLocal();
public static final ThreadLocal<Integer> threadLocal2 = new ThreadLocal();
public static void main(String[] args) {
threadLocal.set(1234);
Integer num1 = threadLocal.get();
System.out.println(num1);
Integer num2 = threadLocal.get();
System.out.println(num2);
threadLocal2.set(111);
System.out.println(threadLocal2.get());
//最后必须remove,避免内存泄露
threadLocal.remove();
threadLocal2.remove();
}
}
ThreadLocal中的set方法以及涉及到的相关方法
public void set(T value) {
Thread t = Thread.currentThread();
//获取当前线程的threadLocals
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
//如果获取不到当前线程的threadlocals成员变量，则新建并将value放入
createMap(t, value);
}
/**
* 获取当前线程的threadlocals
* 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;
}
/**
* 创建ThreadLocalMap,并对当前线程的threadlocals进行赋值
* Create the map associated with a ThreadLocal. Overridden in
* InheritableThreadLocal.
*
* @param t the current thread
* @param firstValue value for the initial entry of the map
*/
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
ThreadLocal静态内部类ThreadLocalMap
/**
* 初始化ThreadLocalMap的构造器
* 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);
}
/**
* 玩当前线程的threadlocals对应的threadLocalMap中赋值
* 用户new的threadlocal的引用地址作为key，value作为value进行赋值构建Entry对象，如果已存在，则进行覆盖
* Set the value associated with key.
*
* @param key the thread local object
* @param value the value to be set
*/
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<ThreadLocal<?>,Object>数组，存放多个threadlocal的数据
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();
}
ThreadLocalMap内部类Entry
/**
* 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继承弱引用，弱引用的特点是在jvm进行gc扫描中直接进行回收
//这种操作既有可能k的引用被回收，而v的值失去所有到达的引用，造成内存泄露,所以在最后必须remove
Entry(ThreadLocal<?> k, Object v) {
super(k);
value = v;
}
}
