ThreadLocal精进篇：子线程类InheritableThreadLocal
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背景
ThreadLocal可以保证在当前运行线程中的变量不被其他并发下的线程共享。
但是如果在代码中需要使用多线程呢？
ThreadLocal是否该如何保证相关子线程下的数据的传递安全性呢？
InheritableThreadLocal给我们提供了一丝可能。
InheritableThreadLocal
InheritableThreadLocal源码
先简单瞄下源码
package java.lang;
import java.lang.ref.*;
public class InheritableThreadLocal<T> extends ThreadLocal<T>
protected T childValue(T parentValue)
return parentValue;
ThreadLocalMap getMap(Thread t)
return t.inheritableThreadLocals;
void createMap(Thread t, T firstValue)
t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
可以很清楚的看到，InheritableThreadLocal是ThreadLcoal的子类。
测试代码
一切以代码为主，我们先上一段代码
package com.chuangyue;
import com.google.common.util.concurrent.ThreadFactoryBuilder;
import java.util.concurrent.*;
/**
* 可继承线程本地测试
*
*/
public class InheritableThreadLocalTest
public static void main(String[] args)
ThreadLocal<String> threadLocal = new ThreadLocal<>();
InheritableThreadLocal<String> inheritableThreadLocal = new InheritableThreadLocal<>();
threadLocal.set(" I'm threadLocal ");
inheritableThreadLocal.set("I'm inheritableThreadLocal");
test01(threadLocal,inheritableThreadLocal);
public static void test01(ThreadLocal<String> threadLocal ,InheritableThreadLocal<String> inheritableThreadLocal)
ThreadFactory namedThreadFactory = new ThreadFactoryBuilder()
.setNameFormat("demo-pool-%d").build();
ExecutorService singleThreadPool = new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<>(1024), namedThreadFactory, new ThreadPoolExecutor.AbortPolicy());
singleThreadPool.execute(()->
System.out.println("========= begin =========");
System.out.println(Thread.currentThread().getName()+" ThreadLocal: "+threadLocal.get());
System.out.println(Thread.currentThread().getName()+" InheritableThreadLocal: "+inheritableThreadLocal.get());
System.out.println("========= end =========");
);
singleThreadPool.shutdown();
这段测试代码里面，只是设置了ThreadLocal和InheritableThreadLocal两个对象，并且对其进行赋值。
然后再在线程中打印对应的对象结果。
输出结果如下
按照正常的理解，应该是两个值都要输出。
但是在这里，我们可以清楚的看到，ThreadLocal输出值为null
这是为什么呢？
还是老规矩，我们看下源码
ThreadLocal源码
package java.lang;
import jdk.internal.misc.TerminatingThreadLocal;
import java.lang.ref.*;
import java.util.Objects;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.function.Supplier;
public class ThreadLocal<T>
private final int threadLocalHashCode = nextHashCode();
/**
* The next hash code to be given out. Updated atomically. Starts at
* zero.
*/
private static AtomicInteger nextHashCode =
new AtomicInteger();
private static final int HASH_INCREMENT = 0x61c88647;
/**
* Returns the next hash code.
*/
private static int nextHashCode()
return nextHashCode.getAndAdd(HASH_INCREMENT);
protected T initialValue()
return null;
public static <S> ThreadLocal<S> withInitial(Supplier<? extends S> supplier)
return new SuppliedThreadLocal<>(supplier);
/**
* Creates a thread local variable.
* @see #withInitial(java.util.function.Supplier)
*/
public ThreadLocal()
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();
boolean isPresent()
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
return map != null && map.getEntry(this) != null;
private T setInitialValue()
T value = initialValue();
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
if (this instanceof TerminatingThreadLocal)
TerminatingThreadLocal.register((TerminatingThreadLocal<?>) this);
return value;
public void set(T value)
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
public void remove()
ThreadLocalMap m = getMap(Thread.currentThread());
if (m != null)
m.remove(this);
/**
* 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);
/**
* Factory method to create map of inherited thread locals.
* Designed to be called only from Thread constructor.
*
* @param parentMap the map associated with parent thread
* @return a map containing the parent's inheritable bindings
*/
static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap)
return new ThreadLocalMap(parentMap);
/**
* Method childValue is visibly defined in subclass
* InheritableThreadLocal, but is internally defined here for the
* sake of providing createInheritedMap factory method without
* needing to subclass the map class in InheritableThreadLocal.
* This technique is preferable to the alternative of embedding
* instanceof tests in methods.
*/
T childValue(T parentValue)
throw new UnsupportedOperationException();
/**
* An extension of ThreadLocal that obtains its initial value from
* the specified @code Supplier.
*/
static final class SuppliedThreadLocal<T> extends ThreadLocal<T>
private final Supplier<? extends T> supplier;
SuppliedThreadLocal(Supplier<? extends T> supplier)
this.supplier = Objects.requireNonNull(supplier);
@Override
protected T initialValue()
return supplier.get();
/**
* 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 initial capacity -- MUST be a power of two.
*/
private static final int INITIAL_CAPACITY = 16;
/**
* The table, resized as necessary.
* table.length MUST always be a power of two.
*/
private Entry[] table;
/**
* The number of entries in the table.
*/
private int size = 0;
/**
* The next size value at which to resize.
*/
private int threshold; // Default to 0
/**
* Set the resize threshold to maintain at worst a 2/3 load factor.
*/
private void setThreshold(int len)
threshold = len * 2 / 3;
/**
* Increment i modulo len.
*/
private static int nextIndex(int i, int len)
return ((i + 1 < len) ? i + 1 : 0);
/**
* Decrement i modulo len.
*/
private static int prevIndex(int i, int len)
return ((i - 1 >= 0) ? i - 1 : len - 1);
/**
* 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);
/**
* Construct a new map including all Inheritable ThreadLocals
* from given parent map. Called only by createInheritedMap.
*
* @param parentMap the map associated with parent thread.
*/
private ThreadLocalMap(ThreadLocalMap parentMap)
Entry[] parentTable = parentMap.table;
int len = parentTable.length;
setThreshold(len);
table = new Entry[len];
for (Entry e : parentTable)
if (e != null)
@SuppressWarnings("unchecked")
ThreadLocal<Object> key = (ThreadLocal<Object>) e.get();
if (key != null)
Object value = key.childValue(e.value);
Entry c = new Entry(key, value);
int h = key.threadLocalHashCode & (len - 1);
while (table[h] != null)
h = nextIndex(h, len);
table[h] = c;
size++;
/**
* Get the entry associated with key. This method
* itself handles only the fast path: a direct hit of existing
* key. It otherwise relays to getEntryAfterMiss. This is
* designed to maximize performance for direct hits, in part
* by making this method readily inlinable.
*
* @param key the thread local object
* @return the entry associated with key, or null if no such
*/
private Entry getEntry(ThreadLocal<?> key)
int i = key.threadLocalHashCode & (table.length - 1);
Entry e = table[i];
if (e != null && e.get() == key)
return e;
else
return getEntryAfterMiss(key, i, e);
/**
* Version of getEntry method for use when key is not found in
* its direct hash slot.
*
* @param key the thread local object
* @param i the table index for key's hash code
* @param e the entry at table[i]
* @return the entry associated with key, or null if no such
*/
private Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e)
Entry[] tab = table;
int len = tab.length;
while (e != null)
ThreadLocal<?> k = e.get();
if (k == key)
return e;
if (k == null)
expungeStaleEntry(i);
else
i = nextIndex(i, len);
e = tab[i];
return null;
/**
* 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();
/**
* 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;
/**
* 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);
/**
* Expunge a stale entry by rehashing any possibly colliding entries
* lying between staleSlot and the next null slot. This also expunges
* any other stale entries encountered before the trailing null. See
* Knuth, Section 6.4
*
* @param staleSlot index of slot known to have null key
* @return the index of the next null slot after staleSlot
* (all between staleSlot and this slot will have been checked
* for expunging).
*/
private int expungeStaleEntry(int staleSlot)
Entry[] tab = table;
int len = tab.length;
// expunge entry at staleSlot
tab[staleSlot].value = null;
tab[staleSlot] = null;
size--;
// Rehash until we encounter null
Entry e;
int i;
for (i = nextIndex(staleSlot, len);
(e = tab[i]) != null;
i = nextIndex(i, len))
ThreadLocal<?> k = e.get();
if (k == null)
e.value = null;
tab[i] = null;
size--;
else
int h = k.threadLocalHashCode & (len - 1);
if (h != i)
tab[i] = null;
// Unlike Knuth 6.4 Algorithm R, we must scan until
// null because multiple entries could have been stale.
while (tab[h] != null)
h = nextIndex(h, len);
tab[h] = e;
return i;
/**
* Heuristically scan some cells looking for stale entries.
* This is invoked when either a new element is added, or
* another stale one has been expunged. It performs a
* logarithmic number of scans, as a balance between no
* scanning (fast but retains garbage) and a number of scans
* proportional to number of elements, that would find all
* garbage but would cause some insertions to take O(n) time.
*
* @param i a position known NOT to hold a stale entry. The
* scan starts at the element after i.
*
* @param n scan control: @code log2(n) cells are scanned,
* unless a stale entry is found, in which case
* @code log2(table.length)-1 additional cells are scanned.
* When called from insertions, this parameter is the number
* of elements, but when from replaceStaleEntry, it is the
* table length. (Note: all this could be changed to be either
* more or less aggressive by weighting n instead of just
* using straight log n. But this version is simple, fast, and
* seems to work well.)
*
* @return true if any stale entries have been removed.
*/
private boolean cleanSomeSlots(int i, int n)
boolean removed = false;
Entry[] tab = table;
int len = tab.length;
do
i = nextIndex(i, len);
Entry e = tab[i];
if (e != null && e.get() == null)
n = len;
removed = true;
i = expungeStaleEntry(i);
while ( (n >>>= 1) != 0);
return removed;
/**
* Re-pack and/or re-size the table. First scan the entire
* table removing stale entries. If this doesn't sufficiently
* shrink the size of the table, double the table size.
*/
private void rehash()
expungeStaleEntries();
// Use lower threshold for doubling to avoid hysteresis
if (size >= threshold - threshold / 4)
resize();
/**
* Double the capacity of the table.
*/
private void resize()
Entry[] oldTab = table;
int oldLen = oldTab.length;
int newLen = oldLen * 2;
Entry[] newTab = new Entry[newLen];
int count = 0;
for (Entry e : oldTab)
if (e != null)
ThreadLocal<?> k = e.get();
if (k == null)
e.value = null; // Help the GC
else
int h = k.threadLocalHashCode & (newLen - 1);
while (newTab[h] != null)
h = nextIndex(h, newLen);
newTab[h] = e;
count++;
setThreshold(newLen);
size = count;
table = newTab;
/**
* Expunge all stale entries in the table.
*/
private void expungeStaleEntries()
Entry[] tab = table;
int len = tab.length;
for (int j = 0; j < len; j++)
Entry e = tab[j];
if (e != null && e.get() == null)
expungeStaleEntry(j);
我们可以很清楚的看到，在ThreadLocal中也存在着以下三个方法：getMap，createMap和chuildValue。
ThreadLocalMap getMap(Thread t)
return t.threadLocals;
void createMap(Thread t, T firstValue)
t.threadLocals = new ThreadLocalMap(this, firstValue);
T childValue(T parentValue)
throw new UnsupportedOperationException();
我们看下InherittableThreadLocal源码中，对重写的这三个方法的代码注解
/**
* 作为创建子线程时父线程值的函数，计算可继承线程局部变量的子线程初始值.
* 此方法在子线程启动之前从父线程内调用.
* <p>
* 此方法仅返回其输入参数，如果需要其他行为，则应重写此方法.
*
* @param parentValue 父线程值
* @return 子线程初始化值
*/
protected T childValue(T parentValue)
return parentValue;
/**
* 获取与ThreadLocal相关的Map对象
*
* @param t 当前线程
*/
ThreadLocalMap getMap(Thread t)
return t.inheritableThreadLocals;
/**
* 创建一个与ThreadLocal相关的map对象.
*
* @param t 当前线程
* @param firstValue 表初始条目值.
*/
void createMap(Thread t, T firstValue)
t.inheritableThreadLocals = new ThreadLocalMap(this, firstValue);
我们直接跟踪下代码
这个是执行thread的时候，发现当前线程直接为空了。
为什么ThreadLocal值为空
这个问题主要出在Thread上。
我们在Thread代码中找到了这两行
public class Thread implements Runnable
...
ThreadLocal.ThreadLocalMap threadLocals = null;
ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;
...每个主线程都会有一个自己的ThreadLocalMap,所以子线程在调用get方法拿值的时候其实访问的是自己的ThreadLocalMap，这个Map和主线程的Map是两个不同的对象，所以肯定是拿不到值的。ThreadLocalMap初始值是null,所以返回就是null了。这个没毛病
那么，inheritableThreadLocals初始化也为null，为什么它就有值呢？
Thread初探
Thread自己怎么玩的
在研究这个之前，我们先看下Thread自己是怎么玩的（初始化）
//构造函数
public Thread(ThreadGroup group, String name)
this(group, null, name, 0);
public Thread(Runnable target, String name)
this(null, target, name, 0);
public Thread(ThreadGroup group, Runnable target, String name,
long stackSize)
this(group, target, name, stackSize, null, true);
...最后它们都甩锅给了它：private Thread
private Thread(ThreadGroup g, Runnable target, String name,
long stackSize, AccessControlContext acc,
boolean inheritThreadLocals)
if (name == null)
throw new NullPointerException("name cannot be null");
this.name = name;
Thread parent = currentThread();
SecurityManager security = System.getSecurityManager();
if (g == null)
/* Determine if it's an applet or not */
/* If there is a security manager, ask the security manager
what to do. */
if (security != null)
g = security.getThreadGroup();
/* If the security manager doesn't have a strong opinion
on the matter, use the parent thread group. */
if (g == null)
g = parent.getThreadGroup();
/* checkAccess regardless of whether or not threadgroup is
explicitly passed in. */
g.checkAccess();
/*
* Do we have the required permissions?
*/
if (security != null)
if (isCCLOverridden(getClass()))
security.checkPermission(
SecurityConstants.SUBCLASS_IMPLEMENTATION_PERMISSION);
g.addUnstarted();
this.group = g;
this.daemon = parent.isDaemon();
this.priority = parent.getPriority();
if (security == null || isCCLOverridden(parent.getClass()))
this.contextClassLoader = parent.getContextClassLoader();
else
this.contextClassLoader = parent.contextClassLoader;
this.inheritedAccessControlContext =
acc != null ? acc : AccessController.getContext();
this.target = target;
setPriority(priority);
if (inheritThreadLocals && parent.inheritableThreadLocals != null)
this.inheritableThreadLocals =
ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
/* Stash the specified stack size in case the VM cares */
this.stackSize = stackSize;
/* Set thread ID */
this.tid = nextThreadID();
。。。我们可以看到，默认情况下，inheritThreadLocals的值是true。也就是设置inheritableThreadLocal默认是可传递的。
ThreadLocal.createInheritedMap
我们可以看到上面的代码中这么一行代码，来进行创建inheritedMap的。
我们继续剖析createInheritedMap源码
在createInheritedMap中，将所有的父线程中的Map的值，使用for的方式全部复制到子线程中。
static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap)
return new ThreadLocalMap(parentMap);
/**
* 构建一个新的ThreadLocalsMap,这个ThreadLocalsMap包含所有parentMap中
* Inheritable ThreadLocals. 它只能被createInheritedMap调用.
*
*/
private ThreadLocalMap(ThreadLocalMap parentMap)
Entry[] parentTable = parentMap.table;
int len = parentTable.length;
setThreshold(len);
table = new Entry[len];
for (Entry e : parentTable)
if (e != null)
@SuppressWarnings("unchecked")
ThreadLocal<Object> key = (ThreadLocal<Object>) e.get();
if (key != null)
Object value = key.childValue(e.value);
Entry c = new Entry(key, value);
int h = key.threadLocalHashCode & (len - 1);
while (table[h] != null)
h = nextIndex(h, len);
table[h] = c;
size++;
总结
所以，Thread类中包含的 threadLocals 和 inheritableThreadLocals 两个变量，inheritableThreadLocals 可自动向子线程中传递的ThreadLocal.ThreadLocalMap。这样，在你进行get()操作的时候，自然就能输出值了。
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京东一面：子线程如何获取父线程 ThreadLocal 的值？我蒙了。。。