Android 开发也要掌握的Java知识 -ThreadLoc
2021-03-15 本文已影响0人
进击的包籽
1.ThreadLocal了解
1.1 基本使用
- 不同线程，使用同一个ThreadLocal，存放数据，互不干扰。
ThreadLocal<String> stringThreadLocal = new ThreadLocal<>();
hreadLocal<Integer> integerThreadLocal = new ThreadLocal<>();
private void threadLocalTest() {
//主线程
stringThreadLocal.set("baozi111");
integerThreadLocal.set(111);
Log.d(TAG, Thread.currentThread().getName() + ":" + stringThreadLocal.get());
Log.d(TAG, Thread.currentThread().getName() + ":" + integerThreadLocal.get());
//子线程
new Thread(new Runnable() {
@Override
public void run() {
stringThreadLocal.set("baozi222");
integerThreadLocal.set(222);
Log.d(TAG, Thread.currentThread().getName() + ":" + stringThreadLocal.get());
Log.d(TAG, Thread.currentThread().getName() + ":" + integerThreadLocal.get());
}
}).start();
}
运行结果
1.2 基础知识
- Android中Handler消息机制非常重要，到处都离不开Handler，每个线程只能有一个Loop，就是利用ThreadLocal。
- ThreadLocal和Synchonized有什么区别呢，Synchonized是利用锁的机制，使得变量或者代码块只能一个线程访问，数据共享，而ThreadLocal就是给每个线程一个变量的副本，每个线程都有，且互相不影响，隔离了多个线程的数据共享。
- ThreadLocal类接口很简单，经常给我调用的只有3个方法，分别是set()、get()、remove()。
1.3 原理
- ThreadLocal本身自己不存储内容，内容由各个线程自己保存，Thread类的 threadLocals，也就是 ThreadLocalMap保存，默认为空，只有在使用时才会初始化，ThreadLocalMap是一个Entry类型数组 ，Entry的key为ThreadLocal，value为保存的内容。每个线程自己保持自己的数据，所以线程之间隔离开了。
记住图好理解
- 上图这个Entry数组里面，不同ThreadLocal，计算的位置不一样，画图只是方便就顺序画了。
2.ThreadLocal源码
2.1 构造方法
- 构造方法啥也没有，那大概率就是在set的时候初始化各种东西了。
/**
* Creates a thread local variable.
* @see #withInitial(java.util.function.Supplier)
*/
public ThreadLocal() {
}
2.2 ThreadLocalMap类
2.2.1 Entry
- 源码可以看到Entry这个类是弱引用的类型。
- key放ThreadLocal，value放存放的内容。
/**
* 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;
}
}
2.2.2 构造方法
- 创建一个长度为16的Entry数组，然后存放的位置用hashcode和15做与运算得到，再保存内容。
/**
* 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 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;
/**
* 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);
}
.
.
}
2.2.3 set(ThreadLocal<?> key, Object value)
- 如果计算的位置已经有内容了，就覆盖，否则就新建一个Entry存放，再判断是否达到阈值，不够就扩容。
/**
* 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();
}
2.2.4 setInitialValue()
- 初始化空Entry，因为有的线程ThreadLocal一开始没有存东西，但调用了get方法，这时候没内容，但还是先占个坑，返回null。
/**
* 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;
}
/**
* 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);
return value;
}
2.2.5 getEntry(ThreadLocal<?> key)
- 利用hashcode找到对应的Entry，如果一次命中找到最好，但扩容过可能不可以一次命中，就要执行 getEntryAfterMiss 方法找。
/**
* 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;
}
2.2.6 整理扩容
- 阈值threshold，为数组容量的2/3。
- 当满足条件if (!cleanSomeSlots(i, sz) && sz >= threshold)，就是没有删除过，且容量使用了2/3时，先执行rehash()就会整理或者扩容。
- 整理完成，如果满足 if (size >= threshold - threshold / 4) 就会触发扩容，扩容2倍。
/**
* Set the resize threshold to maintain at worst a 2/3 load factor.
*/
private void setThreshold(int len) {
//数组长度的2/3
threshold = len * 2 / 3;
}
/**
* 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() {
//整理
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;
//扩容为原来的2倍
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;
}
2.3 set(T value)
- 这个方法就是给当前线程的副本赋设置值，我们在使用时直接在线程里直接使用，在这里会自动切换到当前线程Thread.currentThread()。
- 这里有一个**ThreadLocalMap **，我们可以看到将当前线程存进去了。
- 如果map存在就传入this也就是ThreadLocal，还有需要保存的value，否则创建，传入当前线程和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);
}
2.3.1 set(this, value)
- 直接调用 ThreadLocalMap.set 方法，存储或者替换内容。
2.3.2 createMap(Thread t, T firstValue)
- 创建Map就是调用 ThreadLocalMap构造方法 。
/**
* 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);
}
2.4 get()
- 先获取当前线程的ThreadLocalMap ，然后在ThreadLocalMap 里找对应的Entry，如果找不到就创建一个Entry，占个坑。
/**
* 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() {
//获取当前线程
Thread t = Thread.currentThread();
//获取当前线程的ThreadLocalMap
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();
}
3. 最后
- 理解的如果有错误，希望大家帮忙指出，谢谢~~
