Java IO (4) - Writer

Java IO (4) - Writer

前言

JavaIO一共包括两种，一种是stream，一种是reader/writer，每种又包括in/out，所以一共是四种包。Java 流在处理上分为字符流和字节流。字符流处理的单元为 2 个字节的 Unicode 字符，分别操作字符、字符数组或字符串，而字节流处理单元为 1 个字节，操作字节和字节数组。

Java 内用 Unicode 编码存储字符，字符流处理类负责将外部的其他编码的字符流和 java 内 Unicode 字符流之间的转换。而类 InputStreamReader 和 OutputStreamWriter 处理字符流和字节流的转换。字符流（一次可以处理一个缓冲区）一次操作比字节流（一次一个字节）效率高。

0. 目录

1. Writer
2. BufferedWriter
3. OutputStreamWriter
4. FileWriter

1. Writer

Abstract class for writing to character streams. The only methods that a subclass must implement are write(char[], int, int), flush(), and close(). Most subclasses, however, will override some of the methods defined here in order to provide higher efficiency, additional functionality, or both.

1.1. 主要方法

Writer	append(char c)
Appends the specified character to this writer.

Writer	append(CharSequence csq)
Appends the specified character sequence to this writer.

Writer	append(CharSequence csq, int start, int end)
Appends a subsequence of the specified character sequence to this writer.

abstract void	close()
Closes the stream, flushing it first.

abstract void	flush()
Flushes the stream.

void	write(char[] cbuf)
Writes an array of characters.

abstract void	write(char[] cbuf, int off, int len)
Writes a portion of an array of characters.

void	write(int c)
Writes a single character.

void	write(String str)
Writes a string.

void	write(String str, int off, int len)
Writes a portion of a string.

同Reader类似，本文介绍一下BufferedWriter和OutputStreamWriter，FileWriter。

2. BufferedWriter

Writes text to a character-output stream, buffering characters so as to provide for the efficient writing of single characters, arrays, and strings.

The buffer size may be specified, or the default size may be accepted. The default is large enough for most purposes.

2.1. 构造函数

BufferedWriter(Writer out)
Creates a buffered character-output stream that uses a default-sized output buffer.

BufferedWriter(Writer out, int sz)
Creates a new buffered character-output stream that uses an output buffer
of the given size.

2.2. 主要方法

void	close()
Closes the stream, flushing it first.

void	flush()
Flushes the stream.

void	newLine()
Writes a line separator.

void	write(char[] cbuf, int off, int len)
Writes a portion of an array of characters.

void	write(int c)
Writes a single character.

void	write(String s, int off, int len)
Writes a portion of a String.

2.3. 需要注意的地方

2.3.1. flush方法

需要注意里面的flush方法

flush方法的说明是这样的

Flushes the stream.

也就是说会把内容flush进去，冲进去。

那什么时候用这个呢？答案就是，基本上不用。

先看看flush干了什么事情。

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

就是调用了flushBuffer()方法。

那么close的时候干了什么事情呢？

  public void close()
    throws IOException
  {
    synchronized (this.lock)
    {
      if (this.out == null) {
        return;
      }
      try
      {
        flushBuffer();
      }
      finally
      {
        this.out.close();
        this.out = null;
        this.cb = null;
      }
    }
  }

注意，也调用了flushBuffer()。也就是说，只要你close了，就不用flush了。

而且，write也会flush，其代码如下：

  public void write(int paramInt)
    throws IOException
  {
    synchronized (this.lock)
    {
      ensureOpen();
      if (this.nextChar >= this.nChars) {
        flushBuffer();
      }
      this.cb[(this.nextChar++)] = ((char)paramInt);
    }
  }

那close和write都flush了，flush还有啥用呢？有人这么说：

In other words, relax - just write, write, write and close