Android(安卓)ThreadLocal 源码分析
16lz
2022-01-03
1.为啥 说是Android ThreadLocal ,而不是java ThreadLocal,因为Android 对它进行了 优化.优化地方:内存复用,使用弱引用解决内存泄漏.而且他们处理方式也不同Java 使用类来包裹 key和value的.使用魔数0x61c88647, 计算得到的索引值偶数和奇数之间不断切换.而Android 只是在偶数索引index 存放key ,index+1来存放值.魔数为0x61c88647*2 得到的索引值都是偶数,非常适合它的处理方式.
2.从使用来分析源码.
ThreadLocal
3.ThreadLocal对象的创建:
public ThreadLocal() {} 就一个空构造 ,那么创建对象都做了什么呢,那就看它成员属性了. /** Weak reference to this thread local instance. */ private final Reference> reference = new WeakReference>(this);//弱引用持有它,有利于回收,防止内存泄漏.如果他为null时它所在的存放数组索引的地方将被设置为TOMBSTONE 对象,value所在的地方设置为null,不在持有它对象也有利于回收.下一个ThreadLocal 对象set时.如果找到数组存放的索引,而且在这个索引数组里面的对象为TOMBSTONE将会被替换成这个.从而达到内存复用. /** Hash counter. */ private static AtomicInteger hashCounter = new AtomicInteger(0); /** * Internal hash. We deliberately don't bother with #hashCode(). * Hashes must be even. This ensures that the result of * (hash & (table.length - 1)) points to a key and not a value. * * We increment by Doug Lea's Magic Number(TM) (*2 since keys are in * every other bucket) to help prevent clustering. */ private final int hash = hashCounter.getAndAdd(0x61c88647 * 2);为啥使用0x61c88647 防止集中,为啥*2 因为key 所在的索引为偶数.第一次计算时hash 为零.也就是第一次创建ThreadLocal key 必然在0索引.然后系统已经使用了n次(hashCounter 为静态...).
主线程存放的对象
其中Looper最为熟悉.
3.get()方法
public T get() { // Optimized for the fast path. Thread currentThread = Thread.currentThread();//value 是从线程对象 localValues 成员属性取出来的中,所以不同线程有不同value, 就是有不同的副本. Values values = values(currentThread); if (values != null) { Object[] table = values.table; int index = hash & values.mask;// 尝试从第一次计算hash 得到索引取值,如果key 等于 将执行getAfterMiss 方法.一般都存在第一次计算得到索引的地方 if (this.reference == table[index]) { return (T) table[index + 1]; } } else {//初始化数组 values = initializeValues(currentThread); } return (T) values.getAfterMiss(this); } Object getAfterMiss(ThreadLocal<?> key) { Object[] table = this.table; int index = key.hash & mask; // If the first slot is empty, the search is over. //如果第一次存放的索引都为null ,那么必然没有set过数据 if (table[index] == null) { Object value = key.initialValue();//默认是null // If the table is still the same and the slot is still empty...// 看到这里挺懵圈的,为啥这样判断呢.在同一个线程是串行执行的,不应该table 发生变化才对(并发问题)// 直到看 这句话The table changed during initialValue() 就突然明白,如果继承重写initialValue方法在里面set 是不是有可能发生扩容,扩容时索引可能发生偏移. if (this.table == table && table[index] == null) { table[index] = key.reference; table[index + 1] = value; size++; //扩容或者检查key 是否被回收 cleanUp(); return value; } // The table changed during initialValue().// 当发生扩容时 就要重新遍历索引了 put(key, value); return value; } // Keep track of first tombstone. That's where we want to go back // and add an entry if necessary. int firstTombstone = -1; // Continue search.// 如果不为空 那就继续遍历 而遍历的范围永远都在0-table.length-1 之间,而且必然是偶数 for (index = next(index);; index = next(index)) { Object reference = table[index]; if (reference == key.reference) { return table[index + 1]; //找到就返回 } // If no entry was found... // 这里的逻辑跟上面差不多的 if (reference == null) { Object value = key.initialValue(); // If the table is still the same... if (this.table == table) { // If we passed a tombstone and that slot still // contains a tombstone... if (firstTombstone > -1 && table[firstTombstone] == TOMBSTONE) {//这里内存复用 table[firstTombstone] = key.reference; table[firstTombstone + 1] = value; tombstones--; size++; // No need to clean up here. We aren't filling // in a null slot. return value; } // If this slot is still empty... if (table[index] == null) { table[index] = key.reference; table[index + 1] = value; size++; cleanUp(); return value; } } // The table changed during initialValue(). put(key, value); return value; } if (firstTombstone == -1 && reference == TOMBSTONE) { // Keep track of this tombstone so we can overwrite it.// 为啥不把 table[firstTombstone] = key.reference...这些语句 放到这里来执行呢.万一后面还有 //reference ==key.reference就不就重复了吗 firstTombstone = index; } } }
4.cleanUp():扩容或者回收标记(设置TOMBSTONE)
private void cleanUp() {// 检查是否扩容 if (rehash()) { // If we rehashed, we needn't clean up (clean up happens as // a side effect). return; } //数量为0 那就没有必要进行回收标记了 if (size == 0) { // No live entries == nothing to clean. return; } // Clean log(table.length) entries picking up where we left off // last time.//这里要从上一次的位置开始检查,为什么呢 因为遍历次数为log2Table.length ,不能完全遍历完,索引需要记录上一次位置才能 完全遍历完. int index = clean; Object[] table = this.table; for (int counter = table.length; counter > 0; counter >>= 1, index = next(index)) { Object k = table[index]; if (k == TOMBSTONE || k == null) { //已经标记跳过 continue; // on to next entry } // The table can only contain null, tombstones and references. @SuppressWarnings("unchecked") Reference> reference = (Reference>) k; if (reference.get() == null) { // This thread local was reclaimed by the garbage collector. table[index] = TOMBSTONE; //有利于回收 table[index + 1] = null; tombstones++; size--; } } // Point cursor to next index. clean = index;//记录 } private boolean rehash() { if (tombstones + size < maximumLoad) { return false; } int capacity = table.length >> 1; int newCapacity = capacity; //当数量大于四分之一时 在扩容两倍 .而当数量大于2分之1 getAndAdd() 才会获得 0索引,后面获取的索引会跟之前一样(测试过).所以1/3 或者1/4 扩容都可以.也就是说在1/3 或者1/4之前获取 索引值是不冲突 ,在set 时用遍历有点想不通. if (size > (capacity >> 1)) { // More than 1/2 filled w/ live entries. // Double size. newCapacity = capacity * 2; } Object[] oldTable = this.table; // Allocate new table. initializeTable(newCapacity); // We won't have any tombstones after this. this.tombstones = 0; // If we have no live entries, we can quit here. if (size == 0) { return true; } // Move over entries. //既然索引不冲突 向后或者向前遍历都没有关系 for (int i = oldTable.length - 2; i >= 0; i -= 2) { Object k = oldTable[i]; if (k == null || k == TOMBSTONE) { // Skip this entry. continue; //这里回收标记已经不用管了,因为创建了新的数组,所以上面this.tombstones = 0 } // The table can only contain null, tombstones and references. @SuppressWarnings("unchecked") Reference> reference = (Reference>) k; ThreadLocal<?> key = reference.get(); if (key != null) { // Entry is still live. Move it over. add(key, oldTable[i + 1]); // 找到位置添加进去就行了 } else { // The key was reclaimed. size--; } } return true; }
5.set(...) 方法:
直接贴vaules.put.. void put(ThreadLocal<?> key, Object value) { cleanUp();//说过 int firstTombstone = -1; //getAfter 部分代码差不多 ,唯一比较不理解的是既然索引不冲突 ,是否直接用 key.hash & mask就得了?为啥还遍历? for (int index = key.hash & mask;; index = next(index)) { Object k = table[index]; if (k == key.reference) { // Replace existing entry. table[index + 1] = value; return; } if (k == null) { if (firstTombstone == -1) { // Fill in null slot. table[index] = key.reference; table[index + 1] = value; size++; return; } // Go back and replace first tombstone. table[firstTombstone] = key.reference; table[firstTombstone + 1] = value; tombstones--; size++; return; } // Remember first tombstone. if (firstTombstone == -1 && k == TOMBSTONE) { firstTombstone = index; } } }
ps.终于知道为啥要遍历了,因为 static AtomicInteger hashCounter 那么 在另一个线程创建多个Threadlocal 对象,在返回主线程创建就可能出现索引冲突.怪不得自己计算的索引值跟上面的图不一样.
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