【java_基础深入】源码分析 BufferedOutputStream.close() flush() Socket.shutdownOutput()

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本文链接： https://blog.csdn.net/chenghan_yang/article/details/101082377

具体问题

bos.未flush() 会造成 Socket 的 阻塞
解决办法： 用 flush 或者 Socket.shutdownOutput()

BufferedOutputStream.close() flush() 和 Socket.shutdownOutput

BufferedOuputStream 成员变量 和 flush()

• BufferedOutputStream 内层被包装类 FilterOutputStream

  /**
   * BufferedOutputStream 继承至 FilterOutputStream
   * 
   */
  public class BufferedOutputStream extends FilterOutputStream {}
  
  public class FilterOutputStream extends OutputStream {
      // 最底层的被包装类 OutputStream
       protected OutputStream out;
      
  }
  

  BufferedOutputStream 的 out 成员变量指的就是被FilterOutputStream包装的 OutputStream

• 构造方法

  public class BufferedOutputStream extends FilterOutputStream {
  	// 接收一个 OutputStream 及其子类 传递给带双参的构造函数
      public BufferedOutputStream(OutputStream out) {
          	// 不指定 buffered 的大小则默认为8192字节
              this(out, 8192);
      }
  
  	// 带双参构造函数, 调用父类的
      public BufferedOutputStream(OutputStream out, int size) {
          super(out);
          if (size <= 0) {
              throw new IllegalArgumentException("Buffer size <= 0");
          }
          buf = new byte[size];
      }
  }
  

  • BufferedOuputStream.flush()

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

    • **flush()**调用本类的 **flushBuffer() **+ out.flush()

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

• write(byte b[], int off, int len)) 方法

      /**
       * 内置的 buffered 桶， 用于装数据
       */
      protected byte buf[];
  
  	/**
       * @param out 内部包装的 OutputStream
       */
      public BufferedOutputStream(OutputStream out) {
          this(out, 8192);
      }
  

  @Override
      public synchronized void write(byte b[], int off, int len) throws IOException {
          // 第一次装的数据已经满了, 清空buffer的数据再用out去写
          if (len >= buf.length) {
              flushBuffer();
              out.write(b, off, len);
              return;
          }
          // 只有 buffered 装满了，才清空 buffer 的数据
          if (len > buf.length - count) {
              flushBuffer();
          }
          // 否则先将数据写入buf内，等待下次写数据
          System.arraycopy(b, off, buf, count, len);
          count += len;
      }
  

  综上 : BufferedOutputStream 是 FilterOutputStream 的包装类，

  ​ FilterOutputStream 是 OutputStream 的包装类

  ---

BufferedOutputStream .close() 和 Socket.shutdownOutput()

• BufferedOutputStream.close() 源码

      @Override
      public void close() throws IOException {
          if (closed) {
              return;
          }
          closed = true;
  
          Throwable flushException = null;
          try {
              flush();
          } catch (Throwable e) {
              flushException = e;
              throw e;
          } finally {
              if (flushException == null) {
                  out.close();
              } else {
                  try {
                      out.close();
                      ...
                  }
              }
          }
      }
  

  BufferedOutputStream 内部调用了底层包装类 OuptStream.close()

• JDK8及之前释放资源使用 close() – 不安全

      /**
       *  关闭流直接关闭socket
       */
  	public void close() throws IOException {
          // Prevent recursion. See BugId 4484411
          if (closing)
              return;
          closing = true;
          if (socket != null) {
              if (!socket.isClosed())
                  socket.close();
          } else
              impl.close();
          closing = false;
      }
  

• JDK9新特性：Socket.shutdownOutput()

      public void shutdownOutput() throws IOException
      {
          if (isClosed())
              throw new SocketException("Socket is closed");
          if (!isConnected())
              throw new SocketException("Socket is not connected");
          if (isOutputShutdown())
              throw new SocketException("Socket output is already shutdown");
          getImpl().shutdownOutput();
          shutOut = true;
      } 
  

  • 能够单方向得关闭流