一、什么是ThreadLocal
-
ThreadLocal是Java中的一个线程局部变量工具类，它提供了线程级别的
变量存储和访问操作，每个线程都有自己独立的ThreadLocal实例。 -
一个ThreadLocal对应线程一个局部变量，一个线程可以有多个ThreadLocal，对应多个变量
二、ThreadLocal解决什么问题
- 多线程并发场景可以使用ThreadLocal创建线程独有变量，保证线程数据隔离
- 实现线程范围内的数据共享，可不通过方法参数传递。解决深层次调用的参数传递问题
三、ThreadLocal源码分析
ThreadLocal的实现原理涉及到三个关键的类：ThreadLocal、Thread和ThreadLocalMap。
1. Thread 类（只提取了关键代码解析）
每个线程都有一个Thread对象，其中包含一个ThreadLocalMap对象，用于存储该线程的线程局部变量。在Thread类中，通过ThreadLocal.ThreadLocalMap字段实现对线程局部变量的存取
public
class Thread implements Runnable {
/* ThreadLocal values pertaining to this thread. This map is maintained
* by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;
}
2. ThreadLocal.ThreadLocalMap类（只提取了关键代码解析）
- ThreadLocalMap是ThreadLocal的内部实现类，是一个自定义的哈希表，用于存储线程局部变量的键值对。
- ThreadLocalMap的键是ThreadLocal实例，值是线程局部变量的值。每个ThreadLocal实例都对应一个键值对，即线程局部变量的值。
public class ThreadLocal<T> {
/**
* 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 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(ThreadLocal<?> k, Object v) {
super(k);
value = v;
}
}
/**
* The table, resized as necessary.
* table.length MUST always be a power of two.
*/
private Entry[] table;
/**
* 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[] 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();
}
/**
* Replace a stale entry encountered during a set operation
* with an entry for the specified key. The value passed in
* the value parameter is stored in the entry, whether or not
* an entry already exists for the specified key.
*
* As a side effect, this method expunges all stale entries in the
* "run" containing the stale entry. (A run is a sequence of entries
* between two null slots.)
*
* @param key the key
* @param value the value to be associated with key
* @param staleSlot index of the first stale entry encountered while
* searching for key.
*/
private void replaceStaleEntry(ThreadLocal<?> key, Object value,
int staleSlot) {
Entry[] tab = table;
int len = tab.length;
Entry e;
// Back up to check for prior stale entry in current run.
// We clean out whole runs at a time to avoid continual
// incremental rehashing due to garbage collector freeing
// up refs in bunches (i.e., whenever the collector runs).
int slotToExpunge = staleSlot;
for (int i = prevIndex(staleSlot, len);
(e = tab[i]) != null;
i = prevIndex(i, len))
if (e.get() == null)
slotToExpunge = i;
// Find either the key or trailing null slot of run, whichever
// occurs first
for (int i = nextIndex(staleSlot, len);
(e = tab[i]) != null;
i = nextIndex(i, len)) {
ThreadLocal<?> k = e.get();
// If we find key, then we need to swap it
// with the stale entry to maintain hash table order.
// The newly stale slot, or any other stale slot
// encountered above it, can then be sent to expungeStaleEntry
// to remove or rehash all of the other entries in run.
if (k == key) {
e.value = value;
tab[i] = tab[staleSlot];
tab[staleSlot] = e;
// Start expunge at preceding stale entry if it exists
if (slotToExpunge == staleSlot)
slotToExpunge = i;
cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
return;
}
// If we didn't find stale entry on backward scan, the
// first stale entry seen while scanning for key is the
// first still present in the run.
if (k == null && slotToExpunge == staleSlot)
slotToExpunge = i;
}
// If key not found, put new entry in stale slot
tab[staleSlot].value = null;
tab[staleSlot] = new Entry(key, value);
// If there are any other stale entries in run, expunge them
if (slotToExpunge != staleSlot)
cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
}
}
内存泄漏问题，为什么Entry类要继承弱引用？
如果 Entry 类不继承 WeakReference<ThreadLocal<?>>，那么在关联的 ThreadLocal对象没有其他强引用时，Entry 对象仍然会被视为可达对象，不会被垃圾回收。这会导致内存泄漏，因为即使 ThreadLocal对象已经没有用处，它和 Entry 对象之间的引用关系仍然存在。
内存泄漏的具体情况如下：
- 创建一个 ThreadLocal 对象，并将其做为某个线程的 ThreadLocalMap 中的key。
- 当不再需要这个 ThreadLocal 对象时，没有手动清除对它的引用。
- 因为 ThreadLocalMap 内部使用的是强引用，所以 ThreadLocal 对象仍然被 ThreadLocalMap 引用（ThreadLocalMap 是通过 ThreadLocal 的静态内部类 ThreadLocalMap 来引用ThreadLocal 的），并间接地引用了对应的 Entry对象（
ThreadLocal对象会作为ThreadLocalMap中的Entry对象的key，因此只要ThreadLocal对象的引用存在，对应的Entry对象就无法被垃圾回收）。- 由于 Entry 对象没有被垃圾回收，其中的值（value）也不会被回收释放，占据着内存。
- 这种情况下，即使线程结束，ThreadLocalMap 中的 Entry 对象仍然保持对 ThreadLocal 对象的引用（
Entry 类中的成员变量持有 ThreadLocal 对象的引用），导致 ThreadLocal 对象无法被垃圾回收。- 在长时间运行的应用中，如果大量 ThreadLocal 对象没有被清理和垃圾回收，就会导致内存泄漏。
因此，通过在 Entry 类中使用 WeakReference<ThreadLocal<?>>，当没有其他强引用时，Entry 对象会被垃圾回收，可以防止内存泄漏
3. ThreadLocal类（只提取了关键代码解析）
-
ThreadLocal是一个泛型类，可通过get()和set()方法对线程局部变量进行存储和读取，实际操作的是线程内部的ThreadLocalMap
-
set()方法：首先获取当前线程，再获取线程下的threadLocals，存在则直接添值，不存在初始化map
public class ThreadLocal<T> {
/**
* Sets the current thread's copy of this thread-local variable
* to the specified value. Most subclasses will have no need to
* override this method, relying solely on the {@link #initialValue}
* method to set the values of thread-locals.
*
* @param value the value to be stored in the current thread's copy of
* this thread-local.
*/
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
/**
* 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;
}
/**
* 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);
}
}
四、ThreadLocal原理总结
1. ThreadLocal的主要原理在于ThreadLocalMap的使用
每个线程在获取和设置线程局部变量时，都会操作自己的ThreadLocalMap。线程局部变量的值存储在ThreadLocalMap中的Entry对象中，每个Entry对象包含一个ThreadLocal实例和该线程局部变量的值。
2. 当调用ThreadLocal的get()方法时
它会先获取当前线程，然后从当前线程的ThreadLocalMap中获取ThreadLocal实例对应的Entry对象。如果Entry对象存在，则返回该Entry对象中存储的线程局部变量的值；如果Entry对象不存在，则返回null。
3. 当调用ThreadLocal的set()方法时
它会先获取当前线程，然后将ThreadLocal实例和要设置的值封装成一个Entry对象，再将该Entry对象存放到当前线程的ThreadLocalMap中。
4. 当线程结束时
ThreadLocalMap中的Entry对象会被垃圾回收器自动清除，从而防止内存泄漏。
通过ThreadLocal的实现原理，我们可以在多线程环境下实现线程间的数据隔离，每个线程都可以独立地访问自己的线程局部变量，互不干扰。这在一些需要在多个线程间共享数据的场景下十分有用，可以提高代码的并发性和线程安全性。
五、优秀的设计总结
1. 弱引用防内存泄漏：
通过在 Entry 类中使用 WeakReference<ThreadLocal<?>>，防止了ThreadLocal与Entry循环引用导致可能出现的内存泄漏
2. 懒初始化：
ThreadLocal内部使用了懒初始化的机制，在第一次获取数据时才会进行初始化。这种方式可以避免不必要的初始化操作，提高性能。
3. 共享数据的隔离：
ThreadLocal可以为每个线程提供独立的数据副本，这样每个线程都可以独立地操作自己的数据，而不会干扰其他线程的数据。这种隔离性有助于避免多线程环境下的数据竞争和线程安全问题。
六、ThreadLocal使用示例
ThreadLocal多线程数据隔离示例
public class ThreadLocalDemo {
private static ThreadLocal<Integer> threadLocal = new ThreadLocal<>();
public static void main(String[] args) {
// 创建两个线程，并启动
Thread thread1 = new Thread(() -> {
// 设置线程1的变量值为1
threadLocal.set(1);
// 获取线程1的变量值
System.out.println("线程1的变量值：" + threadLocal.get());
});
Thread thread2 = new Thread(() -> {
// 设置线程2的变量值为2
threadLocal.set(2);
// 获取线程2的变量值
System.out.println("线程2的变量值：" + threadLocal.get());
});
thread1.start();
thread2.start();
}
}
最后打印结果：
线程1的变量值：1
线程2的变量值：2
由此我们可以得出结果：多线程并发场景，每个线程通过ThreadLocal设置的变量是独有的，并不会被其他线程读取到
ThreadLocal多线程操作同一数据错误示例
public class ThreadLocalSyncExample {
// 创建一个 ThreadLocal 对象
private static ThreadLocal<Integer> threadLocal = new ThreadLocal<>();
public static void main(String[] args) {
// 设置初始值
threadLocal.set(0);
// 创建两个线程并启动
Thread thread1 = new Thread(() -> {
for (int i = 0; i < 5; i++) {
incrementAndPrint();
}
});
Thread thread2 = new Thread(() -> {
for (int i = 0; i < 5; i++) {
incrementAndPrint();
}
});
thread1.start();
thread2.start();
}
private static synchronized void incrementAndPrint() {
// 获取当前线程的变量值并加1
int value = threadLocal.get();
threadLocal.set(value + 1);
System.out.println(Thread.currentThread().getName() + ": " + threadLocal.get());
}
}
扫一扫
2368
被折叠的 条评论
为什么被折叠?
到【灌水乐园】发言
