【Java】关于ByteArrayOutputStream的源码分析

一、问题

1.1、环境
电脑环境：Windows 10；
开发工具：IntelliJ IDEA；
数据库环境：Redis 3.2.100
JDK环境: Jdk1.8；

1.2、问题
ByteArrayOutputStream 是经常用到的字符数组输出流，下面我们就来分析一下该类；JDK 1.8的 ByteArrayOutputStream 源码请见附录；

二、解答

1、概述
ByteArrayOutputStream 实现了OutputStream类，这个缓冲数据，会自动根据写入的数据而增长；数组里的数据还可以通过toByteArray方法来还原成数组或者toString方法来还原成字符串；关闭一个ByteArrayOutputStream是无效的；关闭方法可以在stream被关闭后，再调用

2、具体方法分析：
①、protected byte buf[];
用来缓存数据；

②、 protected int count;
用来统计缓存数据的有效字节数；

③、无参构造

    public ByteArrayOutputStream() {
        this(32);
    }

创建一个新的字节数组流，如果没有给定ByteArrayOutputStream流的缓存容量，系统默认初始化大小为32；并且这个大小可以在适当情况下增长；

④、

    public ByteArrayOutputStream(int size) {
        if (size < 0) {
            throw new IllegalArgumentException("Negative initial size: "
                                               + size);
        }
        buf = new byte[size];
    }

创建一个新的字节数组输出流，专门声明了字节的容量大小；注意，如果容量小于0（可以等于0），会抛出非法的参数异常：无效的初始化大小，并且显示之前声明的错误字节容量；见下图：

⑤、

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

确保容量；如果必须，【字节数组输出流】会增加容量以容纳其最小的元素个数；minCapacity 是最小容量大小；OutOfMemoryError 是超出存储限制错误；如果minCapacity小于0，则会抛出异常；如果不适用当前最大容量时，则会增加容量；增加的校验机制是：如果最小容量减去元素长度大于0，则会增加容量；

这里怎么理解呢，就是写代码的过程中，如果你不能确定当前【字节数组输出流】的容纳元素大小，或者已经确定不能容纳下，则可以调用该方法来确保可容纳；

⑥、

    /**
     * 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;

最大数组容量：Integer封装类的最大值减去8，为啥要减去8呢？因为一些虚拟机会在数组中还原一些头部单词；往往分配的数组回避实际结果大；所以这里减去了8个字节大小；

---

TODO

---

⑦、增长规则：

    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);
    }

入参为最小容量，

• 【旧的容量】大小为【字节数组输出流】中元素的长度；
• 【新的容量】大小为：先将【旧的容量】大小进行左位运算，即扩充1倍，变成原来的2倍；
• 如果【新的容量】小于【最小容量】，则将【新的容量】赋值为最小容量；
• 如果【新的容量】大于数组最大容量，则这里会创建一个大容量的数组，并且将原来【字节数组输出流】中的元素复制到新的大容量数组中去；

⑧、创建大容量【字节数组输出流】

    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }

入参为最小容量大小；如果值小于0，则会抛出超出存储错误；
如果入参的最小容量大于最大的数组容量，则该大数组的容量为Integer的最大值；否则就是当前数组的最大容量；

⑨、单个字节写出方法

    /**
     * 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;
    }

线程同步
入参为int类型的字节；
首先调用ensureCapacity方法，确保容量不会超出存储限制；这里是容量+1，为什么是容量+1呢？因为这个wirte方法时1个字节一个字节写出的；
然后每写出一次，则容量增加1；

⑩、字节数组写出方法；

    /**
     * Writes len bytes from the specified byte array
     * starting at offset off 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;
    }

入参为数组，开始写出的起始点的下标，写出的数组的长度；
如果【起始点】小于0，或者【起始点】大于入参数组的长度，或者需要写出的数组的长度小于0，则会抛出下标越界的错误；

因为我们要在原来的容量的基础上再写入len长度的元素，所以这里要确保容量安全；调用ensureCapacity（）方法；
然后需要调用复制方法，将b数组从b数组的off下标开始，复制len长度到buf数组的末尾，count其实是buf数组最后一个元素后面1个位置的下标；

（11）、copyTo方法

    /**
     * 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 out.write(buf, 0, count).
     *
     * @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);
    }

入参为 OutputStream 输出流；将此【字节数组输出流】写出到一个特定的输出流中，使用了输出流out的write方法；类比管道间水流的接力；或者空中加油机之间接力；

（12）、重置方法

/**
     * Resets the count 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;
    }

将此【字节数组输出流】的长度置为0；因此所有当前的基类的输出流会被丢弃；输出流可以被重新使用；可以重新使用已经被分配的缓存空间；

(13)

    /**
     * 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);
    }

创建一个新分配的byte数组；大小等于当前输出流的大小，并且元素为当前输出流的元素；

14、size()方法

 /**
     * Returns the current size of the buffer.
     *
     * @return  the value of the count field, which is the number
     *          of valid bytes in this output stream.
     * @see     java.io.ByteArrayOutputStream#count
     */
    public synchronized int size() {
        return count;
    }

返回当前输出流缓存的元素的个数大小；（注意是同步的）

（15）、toString()方法

    /**
     * Converts the buffer's contents into a string decoding bytes using the
     * platform's default character set. The length of the new String
     * is a function of the character set, and hence may not be equal to the
     * size of the buffer.
     *
     * 
 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);
    }

将当前输出流中的内容，使用当前平台默认的编码格式转换成一个string字符串；新的String字符串的的长度取决于编码格式的方法；因此可能并不等于当前缓存的大小；
该方法始终会替换格式输入错误的，根据当前平台默认的编码格式下，不可映射的字符序列为默认的值；

（16）toString()方法

    /**
     * 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
     * String is a function of the charset, and hence may not be equal
     * to the length of the byte array.
     *
     * 
 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);
    }

入参为编码格式的名字；如果不支持该编码格式，则会抛出：不支持编码异常；

（17）toString 方法

 /**
     * 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 c in the resulting string is
     * constructed from the corresponding element b in the byte
     * array such that:
     * 
     *     c == (char)(((hibyte & 0xff) << 8) | (b & 0xff))
     * 
     *
     * @deprecated This method does not properly convert bytes into characters.
     * As of JDK 1.1, the preferred way to do this is via the
     * toString(String enc) method, which takes an encoding-name
     * argument, or the toString() 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);
    }

创建一个新分配string字符串；

（18）关闭方法

    /**
     * Closing a ByteArrayOutputStream has no effect. The methods in
     * this class can be called after the stream has been closed without
     * generating an IOException.
     */
    public void close() throws IOException {
    }

关闭ByteArrayOutputStream是没有效果的；这个类的方法只有当stream被关闭后不再创建才可以被调用；

附录：

/*
 * Copyright (c) 1994, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package java.io;

import java.util.Arrays;

/**
 * This class implements an output stream in which the data is
 * written into a byte array. The buffer automatically grows as data
 * is written to it.
 * The data can be retrieved using toByteArray() and
 * toString().
 * 

 * Closing a ByteArrayOutputStream has no effect. The methods in
 * this class can be called after the stream has been closed without
 * generating an IOException.
 *
 * @author  Arthur van Hoff
 * @since   JDK1.0
 */

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() {
        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 len bytes from the specified byte array
     * starting at offset off 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 out.write(buf, 0, count).
     *
     * @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 count 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 count 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 String
     * is a function of the character set, and hence may not be equal to the
     * size of the buffer.
     *
     * 
 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
     * String is a function of the charset, and hence may not be equal
     * to the length of the byte array.
     *
     * 
 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 c in the resulting string is
     * constructed from the corresponding element b in the byte
     * array such that:
     * 
     *     c == (char)(((hibyte & 0xff) << 8) | (b & 0xff))
     * 
     *
     * @deprecated This method does not properly convert bytes into characters.
     * As of JDK 1.1, the preferred way to do this is via the
     * toString(String enc) method, which takes an encoding-name
     * argument, or the toString() 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 ByteArrayOutputStream has no effect. The methods in
     * this class can be called after the stream has been closed without
     * generating an IOException.
     */
    public void close() throws IOException {
    }

}

完毕~

三、总结

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