ByteArrayOutputStream.java

/*
 * Copyright (c) 1994, 2018, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package java.io;

import java.nio.charset.Charset;
import java.util.Arrays;
import java.util.Objects;

/**
 * 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 {@code toByteArray()} and
 * {@code toString()}.
 * <p>
 * Closing a {@code ByteArrayOutputStream} has no effect. The methods in
 * this class can be called after the stream has been closed without
 * generating an {@code IOException}.
 *
 * @author Arthur van Hoff
 * @since 1.0
 */
/*
 * 字节数组输出流
 *
 * 写入的内容会先存入内部字节缓存区，这一点与BufferedOutputStream是相似的。
 * 不同点在于，BufferedOutputStream中包装了输出流，当缓冲区满时，会将其中的数据自动刷新到输出流。
 * 而ByteArrayOutputStream中没有包装输出流，所有写入的数据都会存入内部缓存区，直到被动地调用writeTo后，
 * 才会将内部缓存区的数据转移到指定的输出流。
 */
public class ByteArrayOutputStream extends OutputStream {
    
    /**
     * 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;
    
    /**
     * The buffer where data is stored.
     */
    protected byte[] buf;   // 内部缓冲区
    
    /**
     * The number of valid bytes in the buffer.
     */
    protected int count;    // 记录写入缓冲区的字节数
    
    
    
    /*▼ 构造器 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Creates a new {@code ByteArrayOutputStream}. The buffer capacity is
     * initially 32 bytes, though its size increases if necessary.
     */
    public ByteArrayOutputStream() {
        this(32);
    }
    
    /**
     * Creates a new {@code ByteArrayOutputStream}, with a buffer capacity of
     * the specified size, in bytes.
     *
     * @param size the initial size.
     *
     * @throws IllegalArgumentException if size is negative.
     */
    public ByteArrayOutputStream(int size) {
        if(size<0) {
            throw new IllegalArgumentException("Negative initial size: " + size);
        }
        buf = new byte[size];
    }
    
    /*▲ 构造器 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 写 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Writes the specified byte to this {@code ByteArrayOutputStream}.
     *
     * @param b the byte to be written.
     */
    // 将指定的字节写入到内部缓存区
    public synchronized void write(int b) {
        ensureCapacity(count + 1);
        buf[count] = (byte) b;
        count += 1;
    }
    
    /**
     * Writes the complete contents of the specified byte array
     * to this {@code ByteArrayOutputStream}.
     *
     * @param b the data.
     *
     * @throws NullPointerException if {@code b} is {@code null}.
     * @apiNote This method is equivalent to {@link #write(byte[], int, int)
     * write(b, 0, b.length)}.
     * @since 11
     */
    // 将字节数组b的内容写入到内部缓存区
    public void writeBytes(byte[] b) {
        write(b, 0, b.length);
    }
    
    /**
     * Writes {@code len} bytes from the specified byte array
     * starting at offset {@code off} to this {@code ByteArrayOutputStream}.
     *
     * @param b   the data.
     * @param off the start offset in the data.
     * @param len the number of bytes to write.
     *
     * @throws NullPointerException      if {@code b} is {@code null}.
     * @throws IndexOutOfBoundsException if {@code off} is negative,
     *                                   {@code len} is negative, or {@code len} is greater than
     *                                   {@code b.length - off}
     */
    // 将字节数组b中off处起的len个字节写入到输出流
    public synchronized void write(byte[] b, int off, int len) {
        Objects.checkFromIndexSize(off, len, b.length);
        ensureCapacity(count + len);
        System.arraycopy(b, off, buf, count, len);
        count += len;
    }
    
    /**
     * Writes the complete contents of this {@code ByteArrayOutputStream} to
     * the specified output stream argument, as if by calling the output
     * stream's write method using {@code out.write(buf, 0, count)}.
     *
     * @param out the output stream to which to write the data.
     *
     * @throws NullPointerException if {@code out} is {@code null}.
     * @throws IOException          if an I/O error occurs.
     */
    // 将内部缓冲区的数据写入到指定的输出流中（不会清空内部缓存区）
    public synchronized void writeTo(OutputStream out) throws IOException {
        // 从字节数组buf中选取count个字节，然后将其写入到输出流
        out.write(buf, 0, count);
    }
    
    /*▲ 写 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 杂项 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Closing a {@code ByteArrayOutputStream} has no effect. The methods in
     * this class can be called after the stream has been closed without
     * generating an {@code IOException}.
     */
    // 关闭输出流（此处无效）
    public void close() throws IOException {
    }
    
    /**
     * Resets the {@code count} field of this {@code ByteArrayOutputStream}
     * 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
     */
    // 清空内部字节缓存区
    public synchronized void reset() {
        count = 0;
    }
    
    /**
     * Returns the current size of the buffer.
     *
     * @return the value of the {@code count} field, which is the number
     * of valid bytes in this output stream.
     *
     * @see java.io.ByteArrayOutputStream#count
     */
    // 返回内部字节缓存区中的字节数量
    public synchronized int size() {
        return count;
    }
    
    /*▲ 杂项 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /**
     * 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()
     */
    // 返回内部字节缓冲区中的数据
    public synchronized byte[] toByteArray() {
        return Arrays.copyOf(buf, count);
    }
    
    /**
     * Converts the buffer's contents into a string decoding bytes using the
     * platform's default character set. The length of the new {@code String}
     * 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 1.1
     */
    // 以JVM默认字符集格式解析内部字节缓冲区中的数据，进而构造String
    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}.
     *
     * <p> This method is equivalent to {@code #toString(charset)} that takes a
     * {@link java.nio.charset.Charset charset}.
     *
     * <p> An invocation of this method of the form
     *
     * <pre> {@code
     *      ByteArrayOutputStream b = ...
     *      b.toString("UTF-8")
     *      }
     * </pre>
     *
     * behaves in exactly the same way as the expression
     *
     * <pre> {@code
     *      ByteArrayOutputStream b = ...
     *      b.toString(StandardCharsets.UTF_8)
     *      }
     * </pre>
     *
     * @param charsetName the name of a supported
     *                    {@link java.nio.charset.Charset charset}
     *
     * @return String decoded from the buffer's contents.
     *
     * @throws UnsupportedEncodingException If the named charset is not supported
     * @since 1.1
     */
    // 以charsetName格式的字符集解析内部字节缓冲区中的数据，进而构造String
    public synchronized String toString(String charsetName) throws UnsupportedEncodingException {
        return new String(buf, 0, count, charsetName);
    }
    
    /**
     * Converts the buffer's contents into a string by decoding the bytes using
     * the specified {@link java.nio.charset.Charset charset}. The length of the new
     * {@code String} 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 the 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 charset the {@linkplain java.nio.charset.Charset charset}
     *                to be used to decode the {@code bytes}
     *
     * @return String decoded from the buffer's contents.
     *
     * @since 10
     */
    // 以charset字符集格式解析内部字节缓冲区中的数据，进而构造String
    public synchronized String toString(Charset charset) {
        return new String(buf, 0, count, charset);
    }
    
    /**
     * 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>{@code
     *     c == (char)(((hibyte & 0xff) << 8) | (b & 0xff))
     * }</pre></blockquote>
     *
     * @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 This method does not properly convert bytes into characters.
     * As of JDK&nbsp;1.1, the preferred way to do this is via the
     * {@link #toString(String charsetName)} or {@link #toString(Charset charset)}
     * method, which takes an encoding-name or charset argument,
     * or the {@code toString()} method, which uses the platform's default
     * character encoding.
     */
    // ▶ 9 ※ 过时
    @Deprecated
    public synchronized String toString(int hibyte) {
        return new String(buf, hibyte, 0, count);
    }
    
    
    
    /**
     * 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)}.
     */
    // 确保内部缓存区容量足够大
    private void ensureCapacity(int minCapacity) {
        // overflow-conscious code
        if(minCapacity - buf.length>0) {
            grow(minCapacity);
        }
    }
    
    /**
     * 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
     */
    // 对内部缓存区进行扩容
    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) {
        // overflow
        if(minCapacity<0) {
            throw new OutOfMemoryError();
        }
        return (minCapacity>MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE;
    }
    
}