ByteArrayOutputStream源码分析

public class ByteArrayOutputStream extends OutputStream {    /**     * The buffer where data is stored.     */    protected byte buf[];    /**     * The number of valid bytes in the buffer.     */    protected int count;    /**     * Creates a new byte array output stream. The buffer capacity is     * initially 32 bytes, though its size increases if necessary.     */    public ByteArrayOutputStream() {        // 默认的数组长度为32        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];    }    /**     * Increases the capacity if necessary to ensure that it can hold     * at least the number of elements specified by the minimum     * capacity argument.     *     * @param minCapacity the desired minimum capacity     * @throws OutOfMemoryError if {@code minCapacity < 0}.  This is     * interpreted as a request for the unsatisfiably large capacity     * {@code (long) Integer.MAX_VALUE + (minCapacity - Integer.MAX_VALUE)}.     */    private void ensureCapacity(int minCapacity) {        // overflow-conscious code        if (minCapacity - buf.length > 0)            grow(minCapacity);    }    /**     * The maximum size of array to allocate.     * Some VMs reserve some header words in an array.     * Attempts to allocate larger arrays may result in     * OutOfMemoryError: Requested array size exceeds VM limit     */    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;    /**     * Increases the capacity to ensure that it can hold at least the     * number of elements specified by the minimum capacity argument.     *     * @param minCapacity the desired minimum capacity     */    private void grow(int minCapacity) {        // overflow-conscious code        int oldCapacity = buf.length;        int newCapacity = oldCapacity << 1;        if (newCapacity - minCapacity < 0)            newCapacity = minCapacity;        if (newCapacity - MAX_ARRAY_SIZE > 0)            newCapacity = hugeCapacity(minCapacity);        buf = Arrays.copyOf(buf, newCapacity);    }    private static int hugeCapacity(int minCapacity) {        if (minCapacity < 0) // overflow            throw new OutOfMemoryError();        return (minCapacity > MAX_ARRAY_SIZE) ?            Integer.MAX_VALUE :            MAX_ARRAY_SIZE;    }    /**     * Writes the specified byte to this byte array output stream.     *     * @param   b   the byte to be written.     */    public synchronized void write(int b) {        ensureCapacity(count + 1);        buf[count] = (byte) b;        count += 1;    }    /**     * Writes <code>len</code> bytes from the specified byte array     * starting at offset <code>off</code> to this byte array output stream.     *     * @param   b     the data.     * @param   off   the start offset in the data.     * @param   len   the number of bytes to write.     */    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;    }    /**     * Writes the complete contents of this byte array output stream to     * the specified output stream argument, as if by calling the output     * stream's write method using <code>out.write(buf, 0, count)</code>.     *     * @param      out   the output stream to which to write the data.     * @exception  IOException  if an I/O error occurs.     */    public synchronized void writeTo(OutputStream out) throws IOException {        out.write(buf, 0, count);    }    /**     * Resets the <code>count</code> field of this byte array output     * stream to zero, so that all currently accumulated output in the     * output stream is discarded. The output stream can be used again,     * reusing the already allocated buffer space.     *     * @see     java.io.ByteArrayInputStream#count     */    public synchronized void reset() {        count = 0;    }    /**     * Creates a newly allocated byte array. Its size is the current     * size of this output stream and the valid contents of the buffer     * have been copied into it.     *     * @return  the current contents of this output stream, as a byte array.     * @see     java.io.ByteArrayOutputStream#size()     */    public synchronized byte toByteArray()[] {        return Arrays.copyOf(buf, count);    }    /**     * Returns the current size of the buffer.     *     * @return  the value of the <code>count</code> field, which is the number     *          of valid bytes in this output stream.     * @see     java.io.ByteArrayOutputStream#count     */    public synchronized int size() {        return count;    }    /**     * Converts the buffer's contents into a string decoding bytes using the     * platform's default character set. The length of the new <tt>String</tt>     * is a function of the character set, and hence may not be equal to the     * size of the buffer.     *     * <p> This method always replaces malformed-input and unmappable-character     * sequences with the default replacement string for the platform's     * default character set. The {@linkplain java.nio.charset.CharsetDecoder}     * class should be used when more control over the decoding process is     * required.     *     * @return String decoded from the buffer's contents.     * @since  JDK1.1     */    public synchronized String toString() {        return new String(buf, 0, count);    }    /**     * Converts the buffer's contents into a string by decoding the bytes using     * the named {@link java.nio.charset.Charset charset}. The length of the new     * <tt>String</tt> is a function of the charset, and hence may not be equal     * to the length of the byte array.     *     * <p> This method always replaces malformed-input and unmappable-character     * sequences with this charset's default replacement string. The {@link     * java.nio.charset.CharsetDecoder} class should be used when more control     * over the decoding process is required.     *     * @param      charsetName  the name of a supported     *             {@link java.nio.charset.Charset charset}     * @return     String decoded from the buffer's contents.     * @exception  UnsupportedEncodingException     *             If the named charset is not supported     * @since      JDK1.1     */    public synchronized String toString(String charsetName)        throws UnsupportedEncodingException    {        return new String(buf, 0, count, charsetName);    }    /**     * Creates a newly allocated string. Its size is the current size of     * the output stream and the valid contents of the buffer have been     * copied into it. Each character <i>c</i> in the resulting string is     * constructed from the corresponding element <i>b</i> in the byte     * array such that:     * <blockquote><pre>     *     c == (char)(((hibyte &amp; 0xff) &lt;&lt; 8) | (b &amp; 0xff))     * </pre></blockquote>     *     * @deprecated This method does not properly convert bytes into characters.     * As of JDK&nbsp;1.1, the preferred way to do this is via the     * <code>toString(String enc)</code> method, which takes an encoding-name     * argument, or the <code>toString()</code> method, which uses the     * platform's default character encoding.     *     * @param      hibyte    the high byte of each resulting Unicode character.     * @return     the current contents of the output stream, as a string.     * @see        java.io.ByteArrayOutputStream#size()     * @see        java.io.ByteArrayOutputStream#toString(String)     * @see        java.io.ByteArrayOutputStream#toString()     */    @Deprecated    public synchronized String toString(int hibyte) {        return new String(buf, hibyte, 0, count);    }    /**     * Closing a <tt>ByteArrayOutputStream</tt> has no effect. The methods in     * this class can be called after the stream has been closed without     * generating an <tt>IOException</tt>.     */    public void close() throws IOException {    }}

ByteArrayOutputStream主要注意几点

第一：ByteArrayOutputStream支持自动扩容，每次扩容的大小为原来的一倍。
第二：ByteArrayOutputStream是一个线程安全类。
第三：public synchronized void write(byte b[], int off, int len)，当写入一个数组的时候，并没有使用的OutputStream默认的write，而是通过System.arraycopy(b, off, buf, count, len)实现，通过这种方法可以大大减小运行的时间，提高性能。
第四：ByteArrayOutputStream不需要结束调用close()和flush()。