Java ByteArrayoutputStream转为FileInputStream

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mob64ca1417eedd 2024-09-24 01:52:16

文章标签 java 开发语言 ci 字节数组数据 文章分类 Java 后端开发

文章目录

前言
一、字节数组流介绍

1.字节数组流作用
2.ByteArrayInputStream介绍
3.ByteArrayOutputStream介绍

二、构造方法

1.ByteArrayInputStream构造
2.ByteArrayOutputStream构造

三、read()方法
四、write()方法

1.write()方法
2.扩容机制

5.字节数组流与缓冲流比较

1.构造方法区别
2.BufferedInputStream与ByteArrayInputStream
3.BufferedOutputStream与ByteArrayOutputStream

总结

前言

前几篇文章我们都是讲的如何读写文件数据，但其实有时候我们不需要读取文件，可能几个简单的字符串就能满足我们的读写需求，这里我们就引入了字节数组流

一、字节数组流介绍

1.字节数组流作用

对于要创建临时性文件的程序以及网络数据的传输、数据压缩后的传输等可以提高运行的的效率，可以不用访问磁盘。

流的来源或目的地并不一定是文件，也可以是内存中的一块空间，例如一个字节数组。java.io.ByteArrayInputStream、java.io.ByteArrayOutputStream就是将字节数组当作流输入来源、输出目的地的类。

2.ByteArrayInputStream介绍

ByteArrayInputStream 是字节数组输入流。它继承于InputStream。

public class ByteArrayInputStream extends InputStream

内部存储一个字节数组，或者说，他有一个缓冲区，缓冲区是一个字节数组

/**
 * An array of bytes that was provided
 * by the creator of the stream. Elements {@code buf[0]}
 * through {@code buf[count-1]} are the
 * only bytes that can ever be read from the
 * stream;  element {@code buf[pos]} is
 * the next byte to be read.
 */
protected byte buf[];

这个内部实现和BufferedInputStream一样

3.ByteArrayOutputStream介绍

ByteArrayOutputStream 是字节数组输出流。它继承于OutputStream。

public class ByteArrayOutputStream extends OutputStream

内部同样存储一个 byte 数组，数组会随着数据的不断写入而自动增长。 可使用 toByteArray() 和 toString() 获取数据。

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

二、构造方法

1.ByteArrayInputStream构造

/**
 * Creates a {@code ByteArrayInputStream}
 * so that it  uses {@code buf} as its
 * buffer array.
 * The buffer array is not copied.
 * The initial value of {@code pos}
 * is {@code 0} and the initial value
 * of  {@code count} is the length of
 * {@code buf}.
 *
 * @param   buf   the input buffer.
 */
public ByteArrayInputStream(byte buf[]) {
    this.buf = buf;
    this.pos = 0;
    this.count = buf.length;
}

/**
 * Creates {@code ByteArrayInputStream}
 * that uses {@code buf} as its
 * buffer array. The initial value of {@code pos}
 * is {@code offset} and the initial value
 * of {@code count} is the minimum of {@code offset+length}
 * and {@code buf.length}.
 * The buffer array is not copied. The buffer's mark is
 * set to the specified offset.
 *
 * @param   buf      the input buffer.
 * @param   offset   the offset in the buffer of the first byte to read.
 * @param   length   the maximum number of bytes to read from the buffer.
 */
public ByteArrayInputStream(byte buf[], int offset, int length) {
    this.buf = buf;
    this.pos = offset;
    this.count = Math.min(offset + length, buf.length);
    this.mark = offset;
}

和BufferedInputStream不同的是，ByteArrayInputStream不需要套接一个流实现读取，他要读取的数据来自一个数组，所以构造方法里要接入一个字符数组

2.ByteArrayOutputStream构造

/**
 * 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ByteArrayOutputStream的过程其实就是构造一个字节数组
之后就是对这个数组写入（赋值）

三、read()方法

read是要把buf的数据读出去，可以把这里的buf看作一个外部文件，我们要从这个外部文件里读进来数据

/**
 * Reads the next byte of data from this input stream. The value
 * byte is returned as an {@code int} in the range
 * {@code 0} to {@code 255}. If no byte is available
 * because the end of the stream has been reached, the value
 * {@code -1} is returned.
 * <p>
 * This {@code read} method
 * cannot block.
 *
 * @return  the next byte of data, or {@code -1} if the end of the
 *          stream has been reached.
 */
public synchronized int read() {
    return (pos < count) ? (buf[pos++] & 0xff) : -1;
}

/**
 - Reads up to {@code len} bytes of data into an array of bytes from this
 - input stream.  If {@code pos} equals {@code count}, then {@code -1} is
 - returned to indicate end of file.  Otherwise, the  number {@code k} of
 - bytes read is equal to the smaller of {@code len} and {@code count-pos}.
 - If {@code k} is positive, then bytes {@code buf[pos]} through
 - {@code buf[pos+k-1]} are copied into {@code b[off]} through
 - {@code b[off+k-1]} in the manner performed by {@code System.arraycopy}.
 - The value {@code k} is added into {@code pos} and {@code k} is returned.
 - <p>
 - This {@code read} method cannot block.
 -  12. @param   b     the buffer into which the data is read.
 - @param   off   the start offset in the destination array {@code b}
 - @param   len   the maximum number of bytes read.
 - @return  the total number of bytes read into the buffer, or
 -          {@code -1} if there is no more data because the end of
 -          the stream has been reached.
 - @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}
 */
public synchronized int read(byte b[], int off, int len) {
    Objects.checkFromIndexSize(off, len, b.length);

    if (pos >= count) {
        return -1;
    }

    int avail = count - pos;
    if (len > avail) {
        len = avail;
    }
    if (len <= 0) {
        return 0;
    }
    System.arraycopy(buf, pos, b, off, len);
    pos += len;
    return len;
}

read两种调用方法

read()：如果当前读取到的位置小于buf里的总有效字节长度，返回buf[pos] & 0xff；否则返回-1
read(byte b[], int off, int len)：把数据读进b[]里，从b[]的off开始填入len个字节。如果没有剩余的可读数据，返回-1；如果剩余数据量avail不满足需求的len，返回avail

四、write()方法

1.write()方法

write是要把数据写进buf里，如果把这里的buf看作一个外部文件，我们就是要把数据写进这个文件里

/**
 * Writes the specified byte to this {@code ByteArrayOutputStream}.
 *
 * @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} 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}
 */
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;
}

不难看出，write支持两种输入，写入机制与BufferedOutputStream大体类似，这里就不阐述了。不同的地方是，当BufferedOutputStream里的缓冲区满了的时候，它会调用flushBuffer()输出并清空；而ByteArrayOutputStream的缓冲区满了的时候，他会进行一种扩容的操作，直到全部写入为止，下面讲这种扩容机制。

2.扩容机制

每次write的时候，会调用ensureCapacity(int minCapacity)，检验当前缓冲区内存是否充足

/**
 * 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);
}

当最小需求容量大于buf长度时，调用grow函数

/**
 * 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);
}

可以看出grow先考虑扩容一倍，如果newCapacity仍然小于最小需求量，newCapacity = minCapacity，如果newCapacity 大于 MAX_ARRAY_SIZE（Integer.MAX_VALUE-8），调用hugeCapacity检查需求量是否越栈，如果越栈，抛出异常，如果没有越栈，返回Integer.MAX_VALUE，即newCapacity = Integer.MAX_VALUE

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

5.字节数组流与缓冲流比较

字节数组流与缓冲流的源码较为相似，这里分析一下区别

1.构造方法区别

缓冲流需要套接原始流
字节数组流不需要套接一个原始流，构造的时候只需要一个字节数组

2.BufferedInputStream与ByteArrayInputStream

BufferedInputStream读取时，因为套接了一个输入流，所以缓冲区读完还可以重新从磁盘fill
ByteArrayInputStream不依赖磁盘，他的数据只存在于缓冲区buf里，所以缓冲区读完直接返回-1

3.BufferedOutputStream与ByteArrayOutputStream

BufferedOutputStream会创建一个默认的容器量，capacity = 8192 = 8KB，每次在写之前都会检查buf中是否还有空间可以写入，如果没有就flushBuffer()，缓冲区数据写入outputStream中，清空缓冲区，再继续写入内容，期间不存在扩容操作
ByteArrayOutputStream也会创建一个默认的容器量, capacity = 32 = 32b, 每次在写之前都会检查一遍buf剩余空间是否够用, 如果不够用, 就进行扩容操作，一直等到内容写完, 这些数据都会一直处于内存中

当你资源不足够用时,选择BufferedOutputStream是最佳的选择, 当你选择快速完成一个作业时,可以选择ByteArrayOutputStream之类的输出流

总结

字节数组流就总结到这了，下一个会写数据流，将各种基本数据类型与字节流相连通

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