java.io.ByteArrayOutputStream 源码分析

成员变量 buf是存储数据的缓冲区  count是缓冲区中的有效字节数。

    /**
     * The buffer where data is stored.
     */
    protected byte buf[];

    /**
     * The number of valid bytes in the buffer.
     */
    protected int count;

构造参数 默认值32，也可以指定缓冲区到大小

    /**
     * Creates a new byte array output stream. The buffer capacity is
     * initially 32 bytes, though its size increases if necessary.
     */
    public ByteArrayOutputStream() {
        this(32);
    }

    /**
     * Creates a new byte array output stream, with a buffer capacity of
     * the specified size, in bytes.
     *
     * @param   size   the initial size.
     * @exception  IllegalArgumentException if size is negative.
     */
    public ByteArrayOutputStream(int size) {
        if (size < 0) {
            throw new IllegalArgumentException("Negative initial size: "
                                               + size);
        }
        buf = new byte[size];
    }

检查容量是否够用，不够则进行扩容grow。

    private void ensureCapacity(int minCapacity) {
        // overflow-conscious code
        if (minCapacity - buf.length > 0)
            grow(minCapacity);
    }

根据所需要到缓冲区大小重新设置缓冲区。

    private void grow(int minCapacity) {
        //记录旧到缓冲区大小
        int oldCapacity = buf.length;
        //这里是移位运算 相当于 int newCapacity = oldCapacity X 2;
        int newCapacity = oldCapacity << 1;
        //如果2倍的大小仍然不够，直接将minCapacity设置为缓冲区大小
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity < 0) {
            if (minCapacity < 0) // overflow
                throw new OutOfMemoryError();
            newCapacity = Integer.MAX_VALUE;
        }
        //重新设置缓冲区
        buf = Arrays.copyOf(buf, newCapacity);
    }

写入内存流，逻辑就是先判断一下缓冲区是否够用，不够用就先扩容然后再保存。

    public synchronized void write(int b) {
        ensureCapacity(count + 1);
        buf[count] = (byte) b;
        count += 1;
    }

    public synchronized void write(byte b[], int off, int len) {
        if ((off < 0) || (off > b.length) || (len < 0) ||
            ((off + len) - b.length > 0)) {
            throw new IndexOutOfBoundsException();
        }
        ensureCapacity(count + len);
        System.arraycopy(b, off, buf, count, len);
        count += len;
    }

将缓冲区内容写到输出流当中

     public synchronized void writeTo(OutputStream out) throws IOException {
        out.write(buf, 0, count);
     }

返回缓冲区内容

    public synchronized byte toByteArray()[] {
        return Arrays.copyOf(buf, count);
    }