ThreadLocal的实现是通过在线程内部保存了一个map结构，当前线程使用变量时，获取当前线程内部的map,达到了线程本地变量的目的，下面是对源码加了一些注释，有误请指正。
package com.mr.study.threadlocal;
import java.lang.ref.WeakReference;
import java.util.Objects;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.function.Supplier;
/**
* @author zhanxp
* @version 1.0 2019/8/1
*/
public class MyThreadLocal<T> {
/**
* 用于保存该对象的hashCode，通过计算得到，该算法可以很大程度上避免hash冲突
*/
private final int threadLocalHashCode = nextHashCode();
/**
* 下一个ThreadLocal的hash值
*/
private static AtomicInteger nextHashCode =
new AtomicInteger();
/**
* 每次hash增加的值
*/
private static final int HASH_INCREMENT = 0x61c88647;
/**
* 获取下一个hash值
*
* @return
*/
private static int nextHashCode() {
return nextHashCode.getAndAdd(HASH_INCREMENT);
}
/**
* 在线程没有初始化值的时候返回的初始值
*
* @return
*/
protected T initialValue() {
return null;
}
/**
* 通过supplier函数初始化值
*
* @param supplier
* @param <S>
* @return
*/
public static <S> MyThreadLocal<S> withInitial(Supplier<? extends S> supplier) {
return new MyThreadLocal.SuppliedMyThreadLocal<>(supplier);
}
/**
* Creates a thread local variable.
*
* @see #withInitial(java.util.function.Supplier)
*/
public MyThreadLocal() {
}
/**
* 获取线程本地变量
*
* @return
*/
public T get() {
//获取当前线程
Thread t = Thread.currentThread();
MyThreadLocal.MyThreadLocalMap map = getMap(t);
if (map != null) {
//如果map存在，则直接从map中获取值
MyThreadLocal.MyThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
@SuppressWarnings("unchecked")
T result = (T) e.value;
return result;
}
}
//map不存在或者没值，返回初始化值
return setInitialValue();
}
/**
* 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();
MyThreadLocal.MyThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
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();
MyThreadLocal.MyThreadLocalMap 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() {
MyThreadLocal.MyThreadLocalMap m = getMap(Thread.currentThread());
if (m != null) {
m.remove(this);
}
}
/**
* Get the map associated with a MyThreadLocal. Overridden in
* InheritableMyThreadLocal.
*
* @param t the current thread
* @return the map
*/
MyThreadLocal.MyThreadLocalMap getMap(Thread t) {
//线程内部保存了一个MyThreadLocalMap变量，直接获取该变量，该变量是该线程独有的
return t.threadLocals;
}
/**
* Create the map associated with a MyThreadLocal. Overridden in
* InheritableMyThreadLocal.
*
* @param t the current thread
* @param firstValue value for the initial entry of the map
*/
void createMap(Thread t, T firstValue) {
t.threadLocals = new MyThreadLocal.MyThreadLocalMap(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 MyThreadLocal.MyThreadLocalMap createInheritedMap(MyThreadLocal.MyThreadLocalMap parentMap) {
return new MyThreadLocal.MyThreadLocalMap(parentMap);
}
/**
* Method childValue is visibly defined in subclass
* InheritableMyThreadLocal, but is internally defined here for the
* sake of providing createInheritedMap factory method without
* needing to subclass the map class in InheritableMyThreadLocal.
* This technique is preferable to the alternative of embedding
* instanceof tests in methods.
*/
T childValue(T parentValue) {
throw new UnsupportedOperationException();
}
/**
* An extension of MyThreadLocal that obtains its initial value from
* the specified {@code Supplier}.
*/
static final class SuppliedMyThreadLocal<T> extends MyThreadLocal<T> {
private final Supplier<? extends T> supplier;
SuppliedMyThreadLocal(Supplier<? extends T> supplier) {
this.supplier = Objects.requireNonNull(supplier);
}
@Override
protected T initialValue() {
return supplier.get();
}
}
/**
* MyThreadLocalMap is a customized hash map suitable only for
* maintaining thread local values. No operations are exported
* outside of the MyThreadLocal 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 MyThreadLocalMap {
/**
* The entries in this hash map extend WeakReference, using
* its main ref field as the key (which is always a
* MyThreadLocal 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<MyThreadLocal<?>> {
/**
* The value associated with this MyThreadLocal.
*/
Object value;
Entry(MyThreadLocal<?> 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 MyThreadLocal.MyThreadLocalMap.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).
* MyThreadLocalMaps are constructed lazily, so we only create
* one when we have at least one entry to put in it.
*/
MyThreadLocalMap(MyThreadLocal<?> firstKey, Object firstValue) {
table = new MyThreadLocal.MyThreadLocalMap.Entry[INITIAL_CAPACITY];
int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
table[i] = new MyThreadLocal.MyThreadLocalMap.Entry(firstKey, firstValue);
size = 1;
setThreshold(INITIAL_CAPACITY);
}
/**
* Construct a new map including all Inheritable MyThreadLocals
* from given parent map. Called only by createInheritedMap.
*
* @param parentMap the map associated with parent thread.
*/
private MyThreadLocalMap(MyThreadLocal.MyThreadLocalMap parentMap) {
MyThreadLocal.MyThreadLocalMap.Entry[] parentTable = parentMap.table;
int len = parentTable.length;
setThreshold(len);
table = new MyThreadLocal.MyThreadLocalMap.Entry[len];
for (int j = 0; j < len; j++) {
MyThreadLocal.MyThreadLocalMap.Entry e = parentTable[j];
if (e != null) {
@SuppressWarnings("unchecked")
MyThreadLocal<Object> key = (MyThreadLocal<Object>) e.get();
if (key != null) {
Object value = key.childValue(e.value);
MyThreadLocal.MyThreadLocalMap.Entry c = new MyThreadLocal.MyThreadLocalMap.Entry(key, value);
int h = key.threadLocalHashCode & (len - 1);
while (table[h] != null)
h = nextIndex(h, len);
table[h] = c;
size++;
}
}
}
}
/**
* 获取当前本地变量所对应的entry对象
*
* @param key
* @return
*/
private MyThreadLocal.MyThreadLocalMap.Entry getEntry(MyThreadLocal<?> key) {
//计算所在数组中的未知
int i = key.threadLocalHashCode & (table.length - 1);
MyThreadLocal.MyThreadLocalMap.Entry e = table[i];
if (e != null && e.get() == key) {
//如果数组中当前位置的值存在，并且就是该线程变量，则返回当前值
return e;
} else {
//如果当前为空，或者冲突了，调用该方法找值
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 MyThreadLocal.MyThreadLocalMap.Entry getEntryAfterMiss(MyThreadLocal<?> key, int i, MyThreadLocal.MyThreadLocalMap.Entry e) {
MyThreadLocal.MyThreadLocalMap.Entry[] tab = table;
int len = tab.length;
//如果当前e不为空，且不是当前值，那么是hash冲突了，往后找就可以了，如果当前e为null，则说明不存在当前值，直接返回null
while (e != null) {
//获取threadLocal对象
MyThreadLocal<?> k = e.get();
//如果就是当前值，直接返回
if (k == key) {
return e;
}
//如果当前k为空，说明threadLocal已经没有被引用了，这个时候应该清除k所对应的值，避免内存泄露
if (k == null) {
expungeStaleEntry(i);
} else {
//如果当前k不为空，并且不是当前值，则继续往下找
i = nextIndex(i, len);
}
//当前值等于下一个值
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(MyThreadLocal<?> 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.
MyThreadLocal.MyThreadLocalMap.Entry[] tab = table;
int len = tab.length;
//计算ThreadLocal的hash值
int i = key.threadLocalHashCode & (len - 1);
//初始化值应该填充再数组i位置，如果值已经存在了，则往下找，直到找到为空或当前值为止
for (MyThreadLocal.MyThreadLocalMap.Entry e = tab[i];
e != null;
e = tab[i = nextIndex(i, len)]) {
MyThreadLocal<?> k = e.get();
//如果key找到，那就直接替换掉当前的值
if (k == key) {
e.value = value;
return;
}
//如果key为null，说明threadLocal不在被引用，直接用当前的替换掉就可以了
if (k == null) {
replaceStaleEntry(key, value, i);
return;
}
}
//创建一个新的entry对象，对数组赋值
tab[i] = new MyThreadLocal.MyThreadLocalMap.Entry(key, value);
//操作值加1
int sz = ++size;
//清除空槽，如果没有引用被清楚，那么判断是否超过容量，超过了需要重新hash
if (!cleanSomeSlots(i, sz) && sz >= threshold) {
rehash();
}
}
/**
* Remove the entry for key.
*/
private void remove(MyThreadLocal<?> key) {
MyThreadLocal.MyThreadLocalMap.Entry[] tab = table;
int len = tab.length;
//获取数组中未hash冲突的位置
int i = key.threadLocalHashCode & (len - 1);
for (MyThreadLocal.MyThreadLocalMap.Entry e = tab[i];
e != null;
e = tab[i = nextIndex(i, len)]) {
//如果找到了当前值，则清空当前值，清空引用
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.
* <p>
* 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(MyThreadLocal<?> key, Object value,
int staleSlot) {
MyThreadLocal.MyThreadLocalMap.Entry[] tab = table;
int len = tab.length;
MyThreadLocal.MyThreadLocalMap.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).
//当前位置
int slotToExpunge = staleSlot;
//从当前位置往前找，找到数组值为null后的第一个localThread引用被清除的值
for (int i = prevIndex(staleSlot, len);
(e = tab[i]) != null;
i = prevIndex(i, len)) {
if (e.get() == null) {
slotToExpunge = i;
}
}
//查找当前运行的key或者尾随的一个空槽，找到就停止
for (int i = nextIndex(staleSlot, len);
(e = tab[i]) != null;
i = nextIndex(i, len)) {
MyThreadLocal<?> 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.
// 如果找到当前需要存的值，则直接赋值
if (k == key) {
e.value = value;
//值互换
tab[i] = tab[staleSlot];
//staleSlot位置保存当前要插入的值
tab[staleSlot] = e;
// Start expunge at preceding stale entry if it exists
//如果需要清除的是staleSlot，那么需要清除的下标为i（因为值已经发生了互换）
if (slotToExpunge == staleSlot) {
slotToExpunge = i;
}
//清除key为null的引用操作
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 MyThreadLocal.MyThreadLocalMap.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) {
MyThreadLocal.MyThreadLocalMap.Entry[] tab = table;
int len = tab.length;
// expunge entry at staleSlot
//使当前value里面的值不被引用，可以回收
tab[staleSlot].value = null;
//空出数组中的位置
tab[staleSlot] = null;
//存在的值减1
size--;
// Rehash until we encounter null
MyThreadLocal.MyThreadLocalMap.Entry e;
int i;
//因为有值被删除，需要重新计算hash,至到找到数组中的值为空的为止
for (i = nextIndex(staleSlot, len);
(e = tab[i]) != null;
i = nextIndex(i, len)) {
MyThreadLocal<?> k = e.get();
//如果key为null，则清除引用
if (k == null) {
e.value = null;
tab[i] = null;
size--;
} else {
//如果key不为null，需要重新计算hash，放到数组中正确的位置
int h = k.threadLocalHashCode & (len - 1);
if (h != i) {
//如果算出来的值和i不相等，则把i里面当前的值清空，然后把当前值填入合适的位置
tab[i] = null;
// Unlike Knuth 6.4 Algorithm R, we must scan until
// null because multiple entries could have been stale.
//判断数组h位置的值是否为空，如果不为空，则一直往下找，直到找到为空为止
while (tab[h] != null) {
h = nextIndex(h, len);
}
//赋值
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;
MyThreadLocal.MyThreadLocalMap.Entry[] tab = table;
int len = tab.length;
//循环找是否存在key为null，值不为空的对象，如果存在，则清除
do {
i = nextIndex(i, len);
MyThreadLocal.MyThreadLocalMap.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() {
//先清一下所有引用，删除不必要的内存
expungeStaleEntries();
// Use lower threshold for doubling to avoid hysteresis
//如果当前大小超过容量的3/4，则重新调整大小
if (size >= threshold - threshold / 4) {
resize();
}
}
/**
* Double the capacity of the table.
*/
private void resize() {
MyThreadLocal.MyThreadLocalMap.Entry[] oldTab = table;
int oldLen = oldTab.length;
int newLen = oldLen * 2;
MyThreadLocal.MyThreadLocalMap.Entry[] newTab = new MyThreadLocal.MyThreadLocalMap.Entry[newLen];
int count = 0;
for (int j = 0; j < oldLen; ++j) {
//获取旧的值
MyThreadLocal.MyThreadLocalMap.Entry e = oldTab[j];
if (e != null) {
MyThreadLocal<?> k = e.get();
//如果key的引用一句不存在，触发gc
if (k == null) {
e.value = null; // Help the GC
} else {
//否则计算hash值，保存至
int h = k.threadLocalHashCode & (newLen - 1);
while (newTab[h] != null)
h = nextIndex(h, newLen);
newTab[h] = e;
count++;
}
}
}
//调整新的容量
setThreshold(newLen);
//初始化新的容量大小
size = count;
//初始化新的表大小
table = newTab;
}
/**
* Expunge all stale entries in the table.
*/
private void expungeStaleEntries() {
MyThreadLocal.MyThreadLocalMap.Entry[] tab = table;
int len = tab.length;
//遍历全表，删除不必要的引用，触发gc
for (int j = 0; j < len; j++) {
MyThreadLocal.MyThreadLocalMap.Entry e = tab[j];
if (e != null && e.get() == null) {
expungeStaleEntry(j);
}
}
}
}
}
