8.说说threadlocal原理,使用过程中要注意哪些问题
在ThreadLocal中,有一个叫做ThreadLocalMap的静态内部类, 而这个类中有一个虚引用内部类Entry,Entry中有一个value,value存储的就是我们每一个线程的线程变量。而key,就是当前的threadlocal,在每一个线程进行创建的时候,会有一个init操作,在init的过程中会到用一个createMap方法,将父线程中的table给复制到当前线程中并且重新进行rehash,创建完后会赋值给当前线程对象(Thread)的threadLocals成员变量,在进行get的时候,会通过当前ThreadLocal一个内置的成员变量threadLocalHashCode来确定自身的entry在table中存储的slot位置,然后进行一个链表的遍历,对比ThreadLocal
public class ThreadLocal<T> {
/**
* ThreadLocals rely on per-thread linear-probe hash maps attached
* to each thread (Thread.threadLocals and
* inheritableThreadLocals). The ThreadLocal objects act as keys,
* searched via threadLocalHashCode. This is a custom hash code
* (useful only within ThreadLocalMaps) that eliminates collisions
* in the common case where consecutively constructed ThreadLocals
* are used by the same threads, while remaining well-behaved in
* less common cases.
*/
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);
}
/**
* Returns the current thread's "initial value" for this
* thread-local variable. This method will be invoked the first
* time a thread accesses the variable with the {@link #get}
* method, unless the thread previously invoked the {@link #set}
* method, in which case the {@code initialValue} method will not
* be invoked for the thread. Normally, this method is invoked at
* most once per thread, but it may be invoked again in case of
* subsequent invocations of {@link #remove} followed by {@link #get}.
*
* <p>This implementation simply returns {@code null}; if the
* programmer desires thread-local variables to have an initial
* value other than {@code null}, {@code ThreadLocal} must be
* subclassed, and this method overridden. Typically, an
* anonymous inner class will be used.
*
* @return the initial value for this thread-local
*/
protected T initialValue() {
return null;
}
/**
* Creates a thread local variable. The initial value of the variable is
* determined by invoking the {@code get} method on the {@code Supplier}.
*
* @param <S> the type of the thread local's value
* @param supplier the supplier to be used to determine the initial value
* @return a new thread local variable
* @throws NullPointerException if the specified supplier is null
* @since 1.8
*/
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() {
}
/**
* Returns the value in the current thread's copy of this
* thread-local variable. If the variable has no value for the
* current thread, it is first initialized to the value returned
* by an invocation of the {@link #initialValue} method.
*
* @return the current thread's value of this thread-local
*/
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();
}
/**
* Returns {@code true} if there is a value in the current thread's copy of
* this thread-local variable, even if that values is {@code null}.
*
* @return {@code true} if current thread has associated value in this
* thread-local variable; {@code false} if not
*/
boolean isPresent() {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
return map != null && map.getEntry(this) != null;
}
/**
* Variant of set() to establish initialValue. Used instead
* of set() in case user has overridden the set() method.
*
* @return the initial value
*/
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;
}
/**
* 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);
}
}
/**
* Removes the current thread's value for this thread-local
* variable. If this thread-local variable is subsequently
* {@linkplain #get read} by the current thread, its value will be
* reinitialized by invoking its {@link #initialValue} method,
* unless its value is {@linkplain #set set} by the current thread
* in the interim. This may result in multiple invocations of the
* {@code initialValue} method in the current thread.
*
* @since 1.5
*/
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 (int j = 0; j < len; j++) {
Entry e = parentTable[j];
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 (int j = 0; j < oldLen; ++j) {
Entry e = oldTab[j];
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);
}
}
}
}
这个是源码,太长了,我们来分别分析,从经常使用的,get、set、remove来看吧
get
/**
* 返回当前线程的此线程本地变量的副本中的值。如果该变量对于当前线程没有值,
* 则首先将其初始化为通过调用 initialValue 方法返回的值。
*
* @return 此线程本地的当前线程值
*/
public T get() {
Thread t = Thread.currentThread(); // 获取当前线程
ThreadLocalMap map = getMap(t);
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
......
}
return setInitialValue();
}
在这里面我们看到了ThreadLocalMap,我们再来看看ThreadLocalMap,我们先来看看getMap方法
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}
trheadLocals是啥?
继续
/* 与该线程相关的ThreadLocal值。此映射由ThreadLocal类维护。 */
ThreadLocal.ThreadLocalMap threadLocals = null;
好,我们再来看看ThreadLocalMap
ThreadLocalMap 是一个专门用于维护线程本地值的定制哈希映射。 没有导出到 ThreadLocal 类之外的操作。该类是包私有的,以允许在 Thread 类中声明字段。 为了处理非常大且长时间使用的情况,哈希表条目使用 WeakReferences 作为键。 但是,由于没有使用引用队列,因此仅在表空间不足时才能保证删除过时条目。
在ThreadLocalMap中有一个弱引用的内部类 entry
我们来分析一下
在这里我们先来说说WeakRefrences
也就是说,我们只要继承了WeakReference的类的对象都是弱引用
static class Entry extends WeakReference<ThreadLocal<?>> {
/** The value associated with this ThreadLocal. */
Object value;
Entry(ThreadLocal<?> k, Object v) {
super(k);
value = v;
}
}
我们拿一个ThreadLocal作为键,值是啥?
我们来看看,追溯源代码,我们看到了一个方法
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;
}
值最开始是通过initialValue()方法的得来的
但是
protected T initialValue() {
return null;
}
我们看看注释
/**
* 返回当前线程对于这个线程局部变量的“初始值”。该方法将在线程首次使用 {@link #get} 方法访问变量时调用,
* 除非线程先前调用了 {@link #set} 方法,在这种情况下,将不会为线程调用 {@code initialValue} 方法。
* 通常情况下,该方法对于每个线程最多只会被调用一次,但在后续调用 {@link #remove} 后跟 {@link #get} 的情况下可能会再次调用。
*
* <p>这个实现简单地返回 {@code null};如果程序员希望线程局部变量具有除 {@code null} 之外的初始值,
* 则必须对 {@code ThreadLocal} 进行子类化,并覆盖此方法。通常会使用匿名内部类。
*
* @return 这个线程局部变量的初始值
*/
所以说,entry中的value,存放的就是我们放入的变量,还有一个createMap方法
/**
* 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);
}
毫无难度。
好了,我们再来看看get方法吧
get
public T get() {
Thread t = Thread.currentThread(); // 获取当前线程
ThreadLocalMap map = getMap(t); // 获取当前线程对应的ThreadLcoalMap
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this); // 获取当前线程对应的entry
if (e != null) {
...
}
}
return setInitialValue();
}
但是这个getEntry又是怎么做的呢,我们之前没有在Entry里面存Key啊
ThreadLocalMap.getEntry
/**
* 获取与指定键关联的条目。此方法仅处理快速路径:直接命中现有键。否则,它将中继到getEntryAfterMiss。
* 这旨在最大化直接命中的性能,部分原因是通过使该方法容易内联化。
*
* @param key 线程本地对象
* @return 与键关联的条目,如果没有则返回null
*/
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);
}
threadLocalHashCode又是啥?table又是啥?getEntryAfterMiss又是啥?
/**
* ThreadLocal 类依赖于每个线程附加的线性探测哈希映射
* (Thread.threadLocals 和 inheritableThreadLocals)。ThreadLocal 对象充当键,
* 通过 threadLocalHashCode 进行搜索。这是一个自定义的哈希码
* (仅在 ThreadLocalMaps 中有用),它在常见情况下消除了碰撞,
* 即连续构造的 ThreadLocal 对象在相同线程中使用,同时在较不常见的情况下仍然表现良好。
*/
private final int threadLocalHashCode = nextHashCode();
我们来看看nextHashCode方法
/**
* 下一个要分配的哈希码。原子地更新。从零开始。
*/
private static AtomicInteger nextHashCode =
new AtomicInteger();
/**
* 连续生成的哈希码之间的差异 - 将隐式的顺序线程本地 ID 转换为近乎最佳分布的二次幂大小的表的乘法哈希值。
*/
private static final int HASH_INCREMENT = 0x61c88647;
private static int nextHashCode() {
return nextHashCode.getAndAdd(HASH_INCREMENT);
}
这里可以0x61c88647是用来解决哈希冲突的
private Entry[] table;
老生长谈了,不管是在java,还是在之前分析Redis代码的时候,这个一看就是用与存储键值对的
,而key.threadLocalHashCode & (table.length - 1)就是进行选槽,至于为什么是0x61c88647呢?
注释都告诉你了
(将隐式的顺序线程本地 ID 转换为近乎最佳分布的二次幂大小的表的乘法哈希值。【chatgpt老师的锅,我不背】)简单点说就是为了让哈希码能均匀的分布在2的N次方的数组里
既然都这样了,那么看名字和按照经验来猜都应该知道了呗,可能存在hashCode计算相同的Entry,所以在链表上面继续找
/**
* 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;
}
好了,下面我们来看看get
public T get() {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t); // 获取当前线程ThreaLocal所在的ThreadLocalMap
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this); // 获取当前线程ThreadLocal在Map中存储的Entry
if (e != null) {
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
return setInitialValue(); // 如果说之前没有值存在的化,那么初始化
}
不知道大家看到这里有没有问题,我们的ThreadLocal又是怎么多个存在ThreadLocalMap中的呢?
不用想,肯定和static对象有关,这是第一个,其次,这个map我们在Thread中也有通过threadlocals来映射,那么一定会和Thread类有关
我们来找找看
看到了一个createInheritedMap,下面的我们不管,我们看看createInheritedMap
/**
* 工厂方法,用于创建继承的线程本地变量映射。
* 设计为仅从Thread构造函数中调用。
*
* @param parentMap 与父线程关联的映射
* @return 包含父线程可继承绑定的映射
*/
static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
return new ThreadLocalMap(parentMap);
}
果然,和Trhread有关,别急,我们先看看ThreadLocalMap的构造方法
/**
* 将调整阈值设置为保持最坏情况下的 2/3 负载因子。
*/
private void setThreshold(int len) {
threshold = len * 2 / 3;
}
/**
* 构造一个新映射,其中包含给定父映射的所有可继承的ThreadLocal。
* 仅由createInheritedMap调用。
*
* @param parentMap 与父线程关联的映射。
*/
private ThreadLocalMap(ThreadLocalMap parentMap) {
Entry[] parentTable = parentMap.table; // 获取父亲table
int len = parentTable.length; // 长度复制
setThreshold(len); // 负载因子设置
table = new Entry[len]; // 创建一个新的table
for (int j = 0; j < len; j++) {
Entry e = parentTable[j]; // 对每一个slot进行复制
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); // 进行一次rehash
while (table[h] != null)
h = nextIndex(h, len);
table[h] = c;
size++;
}
}
}
}
我们通过代码朔源,发现ThreadLocalMap是在Thread被init的时候进行创建的,我们再来看一看Thread.init()
private void init(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; // 设置线程名称
Thread parent = currentThread(); // 父线程为当前线程
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(); // 默认找安全管理系的线程组
}
/* If the security doesn't have a strong opinion of the matter
use the parent thread group. */
if (g == null) {
g = parent.getThreadGroup(); // 找父线程所在的线程组
}
}
/* 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(SUBCLASS_IMPLEMENTATION_PERMISSION);
}
}
g.addUnstarted();
this.group = g;
this.daemon = parent.isDaemon(); // 如果父线程是守护线程,那么这个线程也是
this.priority = parent.getPriority(); // 配置和父线程一样
if (security == null || isCCLOverridden(parent.getClass()))
this.contextClassLoader = parent.getContextClassLoader(); // 子线程的类加载器和父线程的一样,确保子线程中对类的加载和父线程是使用的同一个类加载器
else
this.contextClassLoader = parent.contextClassLoader;
this.inheritedAccessControlContext =
acc != null ? acc : AccessController.getContext();
this.target = target; // 子线程的执行的任务
setPriority(priority);
if (inheritThreadLocals && parent.inheritableThreadLocals != null)
this.inheritableThreadLocals =
ThreadLocal.createInheritedMap(parent.inheritableThreadLocals); // 父线程的Map传入
/* Stash the specified stack size in case the VM cares */
this.stackSize = stackSize;
/* Set thread ID */
tid = nextThreadID(); // 计算线程ID
}
基本的原理也就这样,那么删除和修改我也就不多说了
原创声明:本文系作者授权腾讯云开发者社区发表,未经许可,不得转载。
如有侵权,请联系 cloudcommunity@tencent.com 删除。
原创声明:本文系作者授权腾讯云开发者社区发表,未经许可,不得转载。
如有侵权,请联系 cloudcommunity@tencent.com 删除。