java io InputSteam

1. Inputstream的类层次

看看jdk源代码中是如何描述该类的:

   
   
   
   

    
    
    
    
    public
    
    
    
     
    
    
    
    abstract
    
    
    
     
    
    
    
    class
    
    
    
     InputStream 
    
    
    
    implements
    
    
    
     Closeable
   
   
   
   

抽象类不能被实例化, 子类实现或继承该类方法, 所以我们经常看到

   
   
   
   

    
    
    
    
    InputStream in 
    
    
    
    =
    
    
    
     
    
    
    
    new
    
    
    
     FileInputStream(
    
    
    
    "
    
    
    
    /opt/1.txt
    
    
    
    "
    
    
    
    );
   
   
   
   

2. InputStream类定义了字节流的基本操作

我们可以查看第三个read方法的源代码, 可以看出来InputStream是一个字节一个字节的读取的

注意观察for循环里面的字节读取。 因为nio是以块的方式读取, 所以对某些应用场景性能上要优于io

   
   
   
   

    
    
    
    
     
    
    
    
    public
    
    
    
     
    
    
    
    int
    
    
    
     read(
    
    
    
    byte
    
    
    
     b[], 
    
    
    
    int
    
    
    
     off, 
    
    
    
    int
    
    
    
     len) 
    
    
    
    throws
    
    
    
     IOException { 
    
    
    
    //
    
    
    
    .....
    
    
    
     
    
    
    
     
    
    
    
    int
    
    
    
     c 
    
    
    
    =
    
    
    
     read(); 
    
    
    
    if
    
    
    
     (c 
    
    
    
    ==
    
    
    
     
    
    
    
    -
    
    
    
    1
    
    
    
    ) { 
    
    
    
    return
    
    
    
     
    
    
    
    -
    
    
    
    1
    
    
    
    ; } b[off] 
    
    
    
    =
    
    
    
     (
    
    
    
    byte
    
    
    
    )c; 
    
    
    
    int
    
    
    
     i 
    
    
    
    =
    
    
    
     
    
    
    
    1
    
    
    
    ; 
    
    
    
    try
    
    
    
     { 
    
    
    
    for
    
    
    
     (; i 
    
    
    
    <
    
    
    
     len ; i
    
    
    
    ++
    
    
    
    ) { c 
    
    
    
    =
    
    
    
     read(); 
    
    
    
    if
    
    
    
     (c 
    
    
    
    ==
    
    
    
     
    
    
    
    -
    
    
    
    1
    
    
    
    ) { 
    
    
    
    break
    
    
    
    ; } b[off 
    
    
    
    +
    
    
    
     i] 
    
    
    
    =
    
    
    
     (
    
    
    
    byte
    
    
    
    )c; } } 
    
    
    
    catch
    
    
    
     (IOException ee) { } 
    
    
    
    return
    
    
    
     i; }
   
   
   
   

关于read方法，还有一个比较重要的地方，

   
   
   
   

    
    
    
    
    /**
    
    
    
     * Reads the next byte of data from the input stream. The value byte is * returned as an int in the range 0 to 255. * If no byte is available because the end of the stream * has been reached, the value -1 is returned. This method * blocks until input data is available, the end of the stream is detected, * or an exception is thrown. * * <p> A subclass must provide an implementation of this method. * * 
    
    
    
    @return
    
    
    
     the next byte of data, or -1 if the end of the * stream is reached. * 
    
    
    
    @exception
    
    
    
     IOException if an I/O error occurs. 
    
    
    
    */
    
    
    
     
    
    
    
    public
    
    
    
     
    
    
    
    abstract
    
    
    
     
    
    
    
    int
    
    
    
     read() 
    
    
    
    throws
    
    
    
     IOException;
   
   
   
   

大家注意这句话:  blocks until input data is available, the end of the stream is detected

就是说是一直阻塞直到检测到流末尾