javaIO--数据流之IO流与字节流

0、IO流

0.1、IO(Input Output)流的概念

Java中将不同设备之间的数据传输抽象为“流”：Stream
设备指的是：磁盘上的文件，网络连接，另一个主机等等

按流向分：输入流，输出流:都是针对内存来说的

1. 输入流，只能从其中读取数据
2. 输出流，只能把数据放入其中

按每次处理的数据单位分：字节流，字符流
1.字节流：每次处理一个字节
2.字符流：每次处理一个字符

0.2、IO流抽象基类

通常流的分类，如果没有特定指出，都是按操作数据单位来说的

字节流：两个方向
InputStream：输入字节流
OutputStream：输出字节流

字节流继承图：

字符流：两个方向
Reader：输入字符流
Writer：输出字符流

字符流继承图：

1、字节流

1.1字节输出流抽象基类：OutputStream

1.1.1 FileOutputStream实现类

构造方法：（输出流会自动创建输出对象）
FileOutputStream(File file)
　　通过一个File对象创建一个文件输出流对象
FileOutputStream(String name)
　　通过一个字符串构建一个文件输出流对象
FileOutputStream(File file,boolean append)
　　通过文件对象创建文件输出流对象，并指定是否追加
FileOutputStream(String name,boolean append)
　　通过一个字符串构建一个文件输出流对象，并指定是否追加

note:如果没有实际对象，对对象的操作并不会作用于实际文件

写出数据到流对象：
void write(byte[] bytes)
　　一次写一个字节数组的数据
void write(int b)
　　一次写一个字节
void write(byte[] bytes,int off,int len)
　　一次写一个字节数组的一部分

关闭流对象：
void close()
　　关闭此文件输出流并释放与此流有关的所有系统资源

关闭流的两个作用：
1.让流不能再继续使用
2.释放和此流相关的系统资源

1.1.2 FileInputStream实现类

构造方法：（必须要求输入对象事先存在）
FileInputStream(File file)
　　通过File对象创建一个文件输入流对象FileInputStream(String name)
　　通过一个字符串创建一个文件输入流对象 String可以是完整路径字符串
int read()
　　从输入流中读取一个字节
int read(byte[] b)
　　从输入流中读取字节，放到字节数组中，返回字节个数
int read(byte[] b,int off,int len)
　　从输入流中读取字节，放到字节数组中，只放到指定位置（不常用）

1.1.3自带关闭资源的try语句块

为了替换finally臃肿的写法，JDK7出现了自带关闭资源的try语句块,可以不用显式的写finally语句。
用法：
将需要关闭的资源定义或者初始化语句放到try后的括号里，程序会自动关闭这些资源，前提是这些类实现了AutoCloseable接口或者Closeable接口
格式：

try(需要自动关闭的资源声明或者初始化){
    //正常逻辑语句
}

案例：字节读取

1.一次读取一个字节
2.用循环改进读取字节
3.简化循环读取字节
4.一次读取一个字节数组
5.加上异常处理读取字节数组
6.改进异常处理读取字节数组

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;

/*
 * FileInputStream(File name)
 * FileInputStream(String name)
 */
public class FileInputStreamDemo {

    public static void main(String[] args) throws Exception {
        //输入流关联的文件必须事先存在,否则异常,输出流目标文件可以自动创建
        FileInputStream fis = new FileInputStream(new File("a.txt"));

//        FileInputStream fis2 = new FileInputStream("a.txt");

        //一次读取一个字节:
//        int b = fis.read();
//        System.out.println(b);
        /*
        b = fis.read();
        System.out.println(b);
        b = fis.read();
        System.out.println(b);
        b = fis.read();
        System.out.println(b);
        b = fis.read();
        System.out.println(b);
        */
        //用循环改进
        /*
        int b = 0;
        while(true){
            b = fis.read();
            if(b == -1){
                break;
            }else{
                System.out.println(b);
            }
        }
        */
        //最终代码
//        int b = 0;
//        while((b = fis.read()) != -1){
//            System.out.println(b);
//        }

        //一次读取一个字节数组
        /*
        byte[] bys = new byte[1024];
        int len = fis.read(bys);
        System.out.println(len);

        //用字节数组构建字符串
        String str = new String(bys, 0, len);
        System.out.println(str);
        */

        //循环读取数据
        int len = 0;
        byte[] bys = new byte[1024];
        while((len = fis.read(bys) + test()) != -1){
            System.out.print(new String(bys,0,len));
        }

        //释放资源
        fis.close();

    }

    public static int test(){
        System.out.println("方法被调用了");
        return 0;
    }

}

字节读取

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;

/*
 * 加上异常处理的程序
 *
 * 1.7后新特性:自带关闭资源的try块
 *
 */
public class FileInputStreamDemo2 {

    public static void main(String[] args) {
        /*
        //提升作用域
        FileInputStream fis = null;
        try {
            fis = new FileInputStream("a.txt");//此条语句有可能不执行成功

            //
            int len = 0;
            byte[] bys = new byte[1024];
            while((len = fis.read(bys)) != -1){
                System.out.print(new String(bys,0,len));
            }
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }finally{
            //用来释放非内存资源
            //释放资源
            //非空判断
            if(fis != null){
                try {
                    fis.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }

            //如果有多个变量,继续关闭
        }

*/

        //自动关闭资源的try块
        try(
                FileInputStream fis = new FileInputStream("a.txt");
                //
                ){
            //正常的逻辑语句
            int len = 0;
            byte[] bys = new byte[1024];
            while((len = fis.read(bys)) != -1){
                System.out.print(new String(bys,0,len));
            }
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

    }

}

异常处理改进

案例：写出数据到流

import java.io.File;
import java.io.FileOutputStream;

/*
 * FileOutputStream(File name)
 * FileOutputStream(File name,boolean append)
 * FileOutputStream(String name)
 * FileOutputStream(String name,boolean append)
 *
 *    创建对象 --> 使用写出方法写出数据到流 --> 释放资源
 *
 *    写入到文件中的数据.如果使用记事本等程序打开,会经过转码的过程. ( 字节-->字符 )
 */
public class FileOutputStreamDemo1 {

    public static void main(String[] args) throws Exception {
        //
        FileOutputStream fos = new FileOutputStream(new File("a.txt"),true);
        /*
        //
        FileOutputStream fos2 = new FileOutputStream(new File("a.txt"), true);

        //
        FileOutputStream fos3 = new FileOutputStream("a.txt");

        //
        FileOutputStream fos4 = new FileOutputStream("a.txt",true);
*/
        //一次写一个字节
//        fos.write(98);

        //一次写一个字节数组
//        byte[] bys = {97,98,99,100,101,102,103,104};
//        fos.write(bys);

        //一次写一个字节数组的一部分
//        fos.write(bys, 2, 3);

        //实现换行
        fos.write("abc\r\ndef".getBytes());//使用系统默认的字符集编码成字节数组 :

        //释放资源
        fos.close();

        //流关闭之后不能再写数据
//        fos.write(97);

    }

}

写出数据到流

案例：文件的复制

思路：
1.使用源文件创建文件输入流对象
2.使用目标文件创建文件输出流对象
3.复制数据
4.关闭资源

实现同一个项目路径下的文本文件的复制
实现不同路径下的文本文件的复制
实现图片的复制
实现mp3的复制 

1.1.4 自带缓冲区的字节流

一次读取(或者写入)一个字节数组的数据在效率上提升了很多，字节数组实际上相当于一个缓冲区。
Java提供了自带缓冲区的流，不必自己再定义额外的字节数组充当缓冲区了。

自带缓冲区的字节输入流     vs      自带缓冲区的字节输出流
BufferedInputStream                   BufferedOutputStream

package java.io;

public class BufferedOutputStream extends FilterOutputStream {

    protected byte buf[];

    protected int count;

    public BufferedOutputStream(OutputStream out) {
        this(out, 8192);
    }

    public BufferedOutputStream(OutputStream out, int size) {
        super(out);
        if (size <= 0) {
            throw new IllegalArgumentException("Buffer size <= 0");
        }
        buf = new byte[size];
    }

    /** Flush the internal buffer */
    private void flushBuffer() throws IOException {
        if (count > 0) {
            out.write(buf, 0, count);
            count = 0;
        }
    }

    public synchronized void write(int b) throws IOException {
        if (count >= buf.length) {
            flushBuffer();
        }
        buf[count++] = (byte)b;
    }

    public synchronized void write(byte b[], int off, int len) throws IOException {
        if (len >= buf.length) {
            /* If the request length exceeds the size of the output buffer,
               flush the output buffer and then write the data directly.
               In this way buffered streams will cascade harmlessly. */
            flushBuffer();
            out.write(b, off, len);
            return;
        }
        if (len > buf.length - count) {
            flushBuffer();
        }
        System.arraycopy(b, off, buf, count, len);
        count += len;
    }

    public synchronized void flush() throws IOException {
        flushBuffer();
        out.flush();
    }
}

BufferedOutputStream

package java.io;
import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;

public
class BufferedInputStream extends FilterInputStream {

    private static int DEFAULT_BUFFER_SIZE = 8192;

    private static int MAX_BUFFER_SIZE = Integer.MAX_VALUE - 8;

    protected volatile byte buf[];

    private static final
        AtomicReferenceFieldUpdater<BufferedInputStream, byte[]> bufUpdater =
        AtomicReferenceFieldUpdater.newUpdater
        (BufferedInputStream.class,  byte[].class, "buf");

    /**
     * The index one greater than the index of the last valid byte in the buffer.
     * 0<count<buf.length
     */
    protected int count;

    /**
     * The current position in the buffer. This is the index of the next character to be read from the buf[].
     * 0<pos<count.  buf[pos] is the next byte to be supplied as input;
     */
    protected int pos;

    /**
     * The value of the pos field at the time the last
     * mark method was called.
     * This value is always in the range -1 through pos.
     * If there is no marked position in  the input
     * stream, this field is -1. If
     * there is a marked position in the input
     * stream,  then buf[markpos]
     * is the first byte to be supplied as input
     * after a reset operation. If
     * markpos is not -1,
     * then all bytes from positions buf[markpos]
     * through  buf[pos-1] must remain
     * in the buffer array (though they may be
     * moved to  another place in the buffer array,
     * with suitable adjustments to the values
     * of count,  pos,
     * and markpos); they may not
     * be discarded unless and until the difference
     * between pos and markpos
     * exceeds marklimit.
     *
     * @see     java.io.BufferedInputStream#mark(int)
     * @see     java.io.BufferedInputStream#pos
     */
    protected int markpos = -1;

    /**
     * The maximum read ahead allowed after a call to the
     * mark method before subsequent calls to the
     * reset method fail.
     * Whenever the difference between pos
     * and markpos exceeds marklimit,
     * then the  mark may be dropped by setting
     * markpos to -1.
     *
     * @see     java.io.BufferedInputStream#mark(int)
     * @see     java.io.BufferedInputStream#reset()
     */
    protected int marklimit;

    /**
     * Check to make sure that underlying input stream has not been
     * nulled out due to close; if not return it;
     */
    private InputStream getInIfOpen() throws IOException {
        InputStream input = in;
        if (input == null)
            throw new IOException("Stream closed");
        return input;
    }

    /**
     * Check to make sure that buffer has not been nulled out due to
     * close; if not return it;
     */
    private byte[] getBufIfOpen() throws IOException {
        byte[] buffer = buf;
        if (buffer == null)
            throw new IOException("Stream closed");
        return buffer;
    }

    /**
     * Creates a BufferedInputStream and saves its  argument, the input stream in, for later use. An internal buffer array is created and  stored in buf.
     * @param   in   the underlying input stream.
     */
    public BufferedInputStream(InputStream in) {
        this(in, DEFAULT_BUFFER_SIZE);
    }

    /**
     * Creates a BufferedInputStream with the specified buffer size, and saves its  argument, the input stream in, for later use.  An internal  buffer array of length  size is created and stored in buf.
     * @param   in     the underlying input stream.
     * @param   size   the buffer size.
     * @exception IllegalArgumentException if {@code size <= 0}.
     */
    public BufferedInputStream(InputStream in, int size) {
        super(in);
        if (size <= 0) {
            throw new IllegalArgumentException("Buffer size <= 0");
        }
        buf = new byte[size];
    }

    /**
     * Fills the buffer with more data, taking into account
     * shuffling and other tricks for dealing with marks.
     * Assumes that it is being called by a synchronized method.
     * This method also assumes that all data has already been read in,
     * hence pos > count.
     */
    private void fill() throws IOException {
        byte[] buffer = getBufIfOpen();
        if (markpos < 0)
            pos = 0;            /* no mark: throw away the buffer */
        else if (pos >= buffer.length)  /* no room left in buffer */
            if (markpos > 0) {  /* can throw away early part of the buffer */
                int sz = pos - markpos;
                System.arraycopy(buffer, markpos, buffer, 0, sz);
                pos = sz;
                markpos = 0;
            } else if (buffer.length >= marklimit) {
                markpos = -1;   /* buffer got too big, invalidate mark */
                pos = 0;        /* drop buffer contents */
            } else if (buffer.length >= MAX_BUFFER_SIZE) {
                throw new OutOfMemoryError("Required array size too large");
            } else {            /* grow buffer */
                int nsz = (pos <= MAX_BUFFER_SIZE - pos) ?
                        pos * 2 : MAX_BUFFER_SIZE;
                if (nsz > marklimit)
                    nsz = marklimit;
                byte nbuf[] = new byte[nsz];
                System.arraycopy(buffer, 0, nbuf, 0, pos);
                if (!bufUpdater.compareAndSet(this, buffer, nbuf)) {
                    // Can't replace buf if there was an async close.
                    // Note: This would need to be changed if fill()
                    // is ever made accessible to multiple threads.
                    // But for now, the only way CAS can fail is via close.
                    // assert buf == null;
                    throw new IOException("Stream closed");
                }
                buffer = nbuf;
            }
        count = pos;
        int n = getInIfOpen().read(buffer, pos, buffer.length - pos);
        if (n > 0)
            count = n + pos;
    }

    public synchronized int read() throws IOException {
        if (pos >= count) {
            fill();
            if (pos >= count)
                return -1;
        }
        return getBufIfOpen()[pos++] & 0xff;
    }

    private int read1(byte[] b, int off, int len) throws IOException {
        int avail = count - pos;
        if (avail <= 0) {
            /* If the requested length is at least as large as the buffer, and
               if there is no mark/reset activity, do not bother to copy the
               bytes into the local buffer.  In this way buffered streams will
               cascade harmlessly. */
            if (len >= getBufIfOpen().length && markpos < 0) {
                return getInIfOpen().read(b, off, len);
            }
            fill();
            avail = count - pos;
            if (avail <= 0) return -1;
        }
        int cnt = (avail < len) ? avail : len;
        System.arraycopy(getBufIfOpen(), pos, b, off, cnt);
        pos += cnt;
        return cnt;
    }

    public synchronized int read(byte b[], int off, int len)
        throws IOException
    {
        getBufIfOpen(); // Check for closed stream
        if ((off | len | (off + len) | (b.length - (off + len))) < 0) {
            throw new IndexOutOfBoundsException();
        } else if (len == 0) {
            return 0;
        }

        int n = 0;
        for (;;) {
            int nread = read1(b, off + n, len - n);
            if (nread <= 0)
                return (n == 0) ? nread : n;
            n += nread;
            if (n >= len)
                return n;
            // if not closed but no bytes available, return
            InputStream input = in;
            if (input != null && input.available() <= 0)
                return n;
        }
    }

    public synchronized long skip(long n) throws IOException {
        getBufIfOpen(); // Check for closed stream
        if (n <= 0) {
            return 0;
        }
        long avail = count - pos;

        if (avail <= 0) {
            // If no mark position set then don't keep in buffer
            if (markpos <0)
                return getInIfOpen().skip(n);

            // Fill in buffer to save bytes for reset
            fill();
            avail = count - pos;
            if (avail <= 0)
                return 0;
        }

        long skipped = (avail < n) ? avail : n;
        pos += skipped;
        return skipped;
    }

    public synchronized int available() throws IOException {
        int n = count - pos;
        int avail = getInIfOpen().available();
        return n > (Integer.MAX_VALUE - avail)
                    ? Integer.MAX_VALUE
                    : n + avail;
    }

    public synchronized void mark(int readlimit) {
        marklimit = readlimit;
        markpos = pos;
    }

    public synchronized void reset() throws IOException {
        getBufIfOpen(); // Cause exception if closed
        if (markpos < 0)
            throw new IOException("Resetting to invalid mark");
        pos = markpos;
    }

    public boolean markSupported() {
        return true;
    }

    public void close() throws IOException {
        byte[] buffer;
        while ( (buffer = buf) != null) {
            if (bufUpdater.compareAndSet(this, buffer, null)) {
                InputStream input = in;
                in = null;
                if (input != null)
                    input.close();
                return;
            }
            // Else retry in case a new buf was CASed in fill()
        }
    }
}

BufferedInputSream

1.1.5测试各种流在复制文件时的效率

FileInputStream
FileOutputStream
基本字节流（节点流）:
一次读取一个字节；
一次读取一个字节数组的数据

BufferedInputStream
BufferedOutputStream
缓冲区流（包装流）:
一次读取一个字节；
一次读取一个字节数组的数据