Netty源码分析第5章(ByteBuf)----第5节: directArena分配缓冲区概述

Netty源码分析第5章(ByteBuf)---->第5节: directArena分配缓冲区概述

Netty源码分析第五章: ByteBuf

第五节: directArena分配缓冲区概述

上一小节简单分析了PooledByteBufAllocator中, 线程局部缓存和arean的相关逻辑, 这一小节简单分析下directArena分配缓冲区的相关过程

回到newDirectBuffer中:

protected ByteBuf newDirectBuffer(int initialCapacity, int maxCapacity) {PoolThreadCache cache = threadCache.get();PoolArena<ByteBuffer> directArena = cache.directArena;ByteBuf buf;if (directArena != null) { buf = directArena.allocate(cache, initialCapacity, maxCapacity);} else {if (PlatformDependent.hasUnsafe()) {buf = UnsafeByteBufUtil.newUnsafeDirectByteBuf(this, initialCapacity, maxCapacity);} else {buf = new UnpooledDirectByteBuf(this, initialCapacity, maxCapacity);}}return toLeakAwareBuffer(buf);
}

获取了directArena对象之后, 通过allocate方法分配一个ByteBuf, 这里allocate方法是PoolArena类中的方法

跟到allocate方法中:

PooledByteBuf<T> allocate(PoolThreadCache cache, int reqCapacity, int maxCapacity) { PooledByteBuf<T> buf = newByteBuf(maxCapacity); allocate(cache, buf, reqCapacity);return buf;
}

首先通过newByteBuf获得一个ByteBuf对象

再通过allocate方法进行分配, 这里要注意, 这里进行分配的时候是线程私有的directArena进行分配

我们跟到newByteBuf方法中

因为是directArena调用的newByteBuf, 所以这里会进入DirectArena类的newByteBuf中:

protected PooledByteBuf<ByteBuffer> newByteBuf(int maxCapacity) { if (HAS_UNSAFE) { return PooledUnsafeDirectByteBuf.newInstance(maxCapacity);} else {return PooledDirectByteBuf.newInstance(maxCapacity);}
}

因为默认通常是有unsafe对象的, 所以这里会走到这一步中PooledUnsafeDirectByteBuf.newInstance(maxCapacity)

通过静态方法newInstance创建一个PooledUnsafeDirectByteBuf对象

跟到newInstance方法中:

static PooledUnsafeDirectByteBuf newInstance(int maxCapacity) {PooledUnsafeDirectByteBuf buf = RECYCLER.get();buf.reuse(maxCapacity);return buf;
}

这里通过RECYCLER.get()这种方式拿到一个ByteBuf对象, RECYCLER其实是一个对象回收站, 这部分内容会在后面的内容中详细剖析, 这里我们只需要知道, 这种方式能从回收站中拿到一个对象, 如果回收站里没有相关对象, 则创建一个新

因为这里有可能是从回收站中拿出的一个对象, 所以通过reuse进行复用

跟到reuse方法中:

final void reuse(int maxCapacity) {maxCapacity(maxCapacity);setRefCnt(1);setIndex0(0, 0);discardMarks();
}

这里设置了的最大可扩容内存, 对象的引用数量, 读写指针位置都重置为0, 以及读写指针的位置标记也都重置为0

我们回到PoolArena的allocate方法中:

PooledByteBuf<T> allocate(PoolThreadCache cache, int reqCapacity, int maxCapacity) { PooledByteBuf<T> buf = newByteBuf(maxCapacity); allocate(cache, buf, reqCapacity);return buf;
}

拿到了ByteBuf对象, 就可以通过allocate(cache, buf, reqCapacity)方法进行内存分配了

跟到allocate方法中:

private void allocate(PoolThreadCache cache, PooledByteBuf<T> buf, final int reqCapacity) {//规格化final int normCapacity = normalizeCapacity(reqCapacity);if (isTinyOrSmall(normCapacity)) { int tableIdx;PoolSubpage<T>[] table;//判断是不是tintyboolean tiny = isTiny(normCapacity);if (tiny) { // < 512//缓存分配if (cache.allocateTiny(this, buf, reqCapacity, normCapacity)) {return;}//通过tinyIdx拿到tableIdxtableIdx = tinyIdx(normCapacity);//subpage的数组table = tinySubpagePools;} else {if (cache.allocateSmall(this, buf, reqCapacity, normCapacity)) {return;}tableIdx = smallIdx(normCapacity);table = smallSubpagePools;}//拿到对应的节点final PoolSubpage<T> head = table[tableIdx];synchronized (head) {final PoolSubpage<T> s = head.next;//默认情况下, head的next也是自身if (s != head) {assert s.doNotDestroy && s.elemSize == normCapacity;long handle = s.allocate();assert handle >= 0;s.chunk.initBufWithSubpage(buf, handle, reqCapacity);if (tiny) {allocationsTiny.increment();} else {allocationsSmall.increment();}return;}}allocateNormal(buf, reqCapacity, normCapacity);return;}if (normCapacity <= chunkSize) {//首先在缓存上进行内存分配if (cache.allocateNormal(this, buf, reqCapacity, normCapacity)) {//分配成功, 返回return;}//分配不成功, 做实际的内存分配
        allocateNormal(buf, reqCapacity, normCapacity);} else {//大于这个值, 就不在缓存上分配
        allocateHuge(buf, reqCapacity);}
}