IO（四）BufferedWriter和 BufferedReader

IO（四）BufferedWriter和 BufferedReader

BufferReader类是从字符输入流中读取文本并缓冲字符，以便有效的读取字符，数组和行

BufferReader

/** * BufferReader */ public static void inBufferWriter() { File file = new File("abc.txt"); try { Reader reader = new FileReader(file); BufferedReader bufferedReader = new BufferedReader(reader); char[] cs = new char[1]; int len = -1; StringBuilder stringBuilder = new StringBuilder(); while(((len =bufferedReader.read(cs))!= -1)) { stringBuilder.append(new String(cs,0,len)); } bufferedReader.close(); //reader.close(); System.out.println(stringBuilder.toString()); } catch (FileNotFoundException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (IOException e) { // TODO Auto-generated catch block e.printStackTrace(); }}

bufferWriter

/** * bufferWriter */ public static void outBufferWriter() { File file = new File("abc.txt"); try( Writer writer = new FileWriter(file); BufferedWriter bufferedWriter = new BufferedWriter(writer); ){ bufferedWriter.write("BufferWriter测试"); } catch (IOException e) { // TODO Auto-generated catch block e.printStackTrace(); } }

测试结果：

原理：

1.FileWriter底层还是FileOutputStream，通过解码器完成字节类转换为字符流

public FileWriter(File file) throws IOException { super(new FileOutputStream(file)); }//通过解码器完成字节类转换为字符流public OutputStreamWriter(OutputStream out) { super(out); try { se = StreamEncoder.forOutputStreamWriter(out, this, (String)null); } catch (UnsupportedEncodingException e) { throw new Error(e); } }

2.BufferedWriter可以通过传入字符流和指定缓冲区大小对字符流进行增强

//没有传入缓冲区大小则使用默认大小public BufferedWriter(Writer out) { this(out, defaultCharBufferSize); } // 指定缓冲区大小，构造指定大小的字符数组 public BufferedWriter(Writer out, int sz) { super(out); if (sz <= 0) throw new IllegalArgumentException("Buffer size <= 0"); this.out = out; cb = new char[sz]; nChars = sz; nextChar = 0; lineSeparator = java.security.AccessController.doPrivileged( new sun.security.action.GetPropertyAction("line.separator")); }

3.默认缓冲区大小是8kb

private static int defaultCharBufferSize = 8192;

4.read()方法源码

public int read() throws IOException { //线程安全 synchronized (lock) { //判断流是否关闭 ensureOpen(); for (;;) { //使用fill()来读取字符 if (nextChar >= nChars) { fill(); if (nextChar >= nChars) return -1; } //如果涉及跳过字节数和换行符问题 if (skipLF) { skipLF = false; if (cb[nextChar] == '\n') { nextChar++; continue; } } return cb[nextChar++]; } } }

fill()方法

private static final int UNMARKED = -1; private char cb[]; private int nChars, nextChar; private static final int INVALIDATED = -2; private static final int UNMARKED = -1; private int markedChar = UNMARKED; private int readAheadLimit = 0; private void fill() throws IOException { //缓冲区存储起始位置 int dst; //如果true-->则现在确认缓冲区的存储开始位置是0 if (markedChar <= UNMARKED) { /* No mark */ //没有标记，缓冲区初始存储位置是0 dst = 0; } else { /* Marked */ //标记情况下，重新确定起始存储位置 //delta ：要存储的字符实际长度 int delta = nextChar - markedChar; //readAheadLimit：最多可存储的字符长度 //如果delta>=readAheadLimit，则标记无效，存储起始位置从0开始 if (delta >= readAheadLimit) { /* Gone past read-ahead limit: Invalidate mark */ markedChar = INVALIDATED; readAheadLimit = 0; dst = 0; } else { //当readAheadLimit小于缓冲区大小 if (readAheadLimit <= cb.length) { /* Shuffle in the current buffer */ //置于cb[]的前面，cb[]剩余的字节从in里面获取 下一部分直至读取完毕 System.arraycopy(cb, markedChar, cb, 0, delta); markedChar = 0; //起始位置是delta dst = delta; } else { /* Reallocate buffer to accommodate read-ahead limit */ char ncb[] = new char[readAheadLimit]; System.arraycopy(cb, markedChar, ncb, 0, delta); cb = ncb; markedChar = 0; dst = delta; } nextChar = nChars = delta; } } int n; do { //cb：缓冲区 //dst：当前所在的缓冲区的字符数组下标 //cb.length - dst：可以存入的字符 //in.read(cb, dst, cb.length - dst)--->返回每次实际读取的字符数量 // //调用InputStreamReader的方法，实际是调用StreamDecoder的read(char cbuf[], int offset, int length)方法 n = in.read(cb, dst, cb.length - dst); } while (n == 0);//读完--->循环结束 if (n > 0) { nChars = dst + n; nextChar = dst; } }

write

public void write(int c) throws IOException { synchronized (lock) { ensureOpen(); //缓冲区满则刷新缓冲区 if (nextChar >= nChars) flushBuffer(); //否则写入缓冲区 cb[nextChar++] = (char) c; } }

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