Netty-源码分析ByteBuf-slice和retainedSlice使用细节

Posted 2022-07-30 征服.刘华强

Netty-源码分析ByteBuf-slice和retainedSlice使用细节

slice() = slice(buf.readerIndex(), buf.readableBytes())

源码片段，返回原始ByteBuf可读字节的一部分， 修改返回的缓冲区或此缓冲区的内容会影响彼此的内容，他们维护单独的index和makers，此方法不会修改原始缓冲区的readerIndex或writerIndex。

 @Override
    public ByteBuf slice() 
        return slice(readerIndex, readableBytes());
    

 @Override
    public ByteBuf slice(int index, int length) 
        ensureAccessible();
        return new UnpooledSlicedByteBuf(this, index, length);
    

    boolean release0() 
        return unwrap().release();
    

public static void main(String[] args) throws Exception 
		PooledByteBufAllocator allocator = PooledByteBufAllocator.DEFAULT;
		ByteBuf original = allocator.directBuffer(32);
		original.writeByte(1);
		original.writeByte(2);
		original.writeByte(3);

		//相当于original.slice(original.readerIndex(), original.readableBytes());
		ByteBuf sub = original.slice();

		//新缓冲区修改值后互相影响
		sub.setByte(0, 10);
		System.out.println("sub.readByte = " + sub.readByte());
		System.out.println("original.readByte = " + original.readByte());

		//原始缓冲区在读一次
		System.out.println("original.readByte = " + original.readByte());

		//Index互不影响
		System.out.println("sub.readerIndex = " + sub.readerIndex());
		System.out.println("original.readerIndex = " + original.readerIndex());

		//必须释放
		original.release();

	

 

与slice一模一样，只是调用了retain，在使用完毕后需要多释放一次。

  @Override
    public ByteBuf retainedSlice() 
        return slice().retain();
    

    @Override
    public final ByteBuf retain() 
        return retain0();
    

    ByteBuf retain0() 
        unwrap().retain();
        return this;
    

 

在使用完毕后，必须要释放俩次。

	public static void main(String[] args) throws Exception 
		PooledByteBufAllocator allocator = PooledByteBufAllocator.DEFAULT;
		ByteBuf original = allocator.directBuffer(32);
		original.writeByte(1);
		original.writeByte(2);
		original.writeByte(3);

		ByteBuf sub = original.retainedSlice();
		sub.release();
		original.release();

		System.out.println("sub.refCnt = " + sub.refCnt());
		System.out.println("original.refCnt = " + original.refCnt());

 

AbstractPooledDerivedByteBuf源码分析

    final <U extends AbstractPooledDerivedByteBuf> U init(
            AbstractByteBuf unwrapped, ByteBuf wrapped, int readerIndex, int writerIndex, int maxCapacity) 
        //原始缓冲区计数器++
        wrapped.retain(); // Retain up front to ensure the parent is accessible before doing more work.
        parent = wrapped;
        rootParent = unwrapped;

        try 
            maxCapacity(maxCapacity);
            setIndex0(readerIndex, writerIndex); // It is assumed the bounds checking is done by the caller.
            resetRefCnt();

    @Override
    protected final void deallocate() 
        // We need to first store a reference to the parent before recycle this instance. This is needed as
        // otherwise it is possible that the same AbstractPooledDerivedByteBuf is again obtained and init(...) is
        // called before we actually have a chance to call release(). This leads to call release() on the wrong parent.
        ByteBuf parent = this.parent;
        recyclerHandle.recycle(this);
        //子缓冲区释放会 减少原始缓冲区计数器
        parent.release();