Java-IO之BufferedWriter（字符缓冲输出流）

BufferedWriter是字符缓冲输出流，继承于Writer，作用是为其他字符输出流添加一些缓冲功能。

BufferedWriter主要的函数列表：

BufferedWriter(Writer out)
BufferedWriter(Writer out, int sz)

void    close()                              // 关闭此流，但要先刷新它。
void    flush()                              // 刷新该流的缓冲。
void    newLine()                            // 写入一个行分隔符。
void    write(char[] cbuf, int off, int len) // 写入字符数组的某一部分。
void    write(int c)                         // 写入单个字符。
void    write(String s, int off, int len)    // 写入字符串的某一部分。

BufferedWriter的示代码：

public class BufferedWriterTest {

    private static final int LEN = 5;
    // 对应英文字母“abcdefghijklmnopqrstuvwxyz”
    //private static final char[] ArrayLetters = "abcdefghijklmnopqrstuvwxyz";
    private static final char[] ArrayLetters = new char[] {'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'};

    public static void main(String[] args) {
        testBufferedWriter() ;
    }

/**
     * BufferedWriter的API测试函数
*/
private static void testBufferedWriter() {

// 创建“文件输出流”对应的BufferedWriter
        // 它对应缓冲区的大小是16，即缓冲区的数据>=16时，会自动将缓冲区的内容写入到输出流。
try {
            File file = new File("bufferwriter.txt");
BufferedWriter out =
new BufferedWriter(
new FileWriter(file));

// 将ArrayLetters数组的前10个字符写入到输出流中
out.write(ArrayLetters, 0, 10);
// 将“换行符\n”写入到输出流中
out.write('\n');

out.flush();
//readUserInput() ;

out.close();
} catch (FileNotFoundException e) {
           e.printStackTrace();
} catch (SecurityException e) {
           e.printStackTrace();
} catch (IOException e) {
           e.printStackTrace();
}
    }

/**
     * 读取用户输入
*/
private static void readUserInput() {
        System.out.println("please input a text:");
Scanner reader=new Scanner(System.in);
// 等待一个输入
String str = reader.next();
System.out.printf("the input is : %s\n", str);
}
}

结果：

文件里写入：abcdefghij

基于JDK8的BufferedWriter的源码：

public class BufferedWriter extends Writer {

private Writer out;

    private char cb[];//缓冲字符数组
// nChars 是cb缓冲区中字符的总的个数
// nextChar 是下一个要读取的字符在cb缓冲区中的位置
private int nChars, nextChar;

    private static int defaultCharBufferSize = 8192;//默认大小是8K

/**
     * Line separator string.  This is the value of the line.separator
     * property at the moment that the stream was created.
     */
private String lineSeparator;

/**
     * Creates a buffered character-output stream that uses a default-sized
     * output buffer.
     *
     * @param  out  A Writer
     */
//构造函数
public BufferedWriter(Writer out) {
this(out, defaultCharBufferSize);
}

/**
     * Creates a new buffered character-output stream that uses an output
     * buffer of the given size.
     *
     * @param  out  A Writer
     * @param  sz   Output-buffer size, a positive integer
     *
     * @exception  IllegalArgumentException  If {@code sz <= 0}
     */
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"));
}

/** Checks to make sure that the stream has not been closed */
//检查流没有被关闭
private void ensureOpen() throws IOException {
if (out == null)
throw new IOException("Stream closed");
}

/**
     * Flushes the output buffer to the underlying character stream, without
     * flushing the stream itself.  This method is non-private only so that it
     * may be invoked by PrintStream.
     */
//刷新流，最后用本地方法写
void flushBuffer() throws IOException {
synchronized (lock) {
            ensureOpen();
            if (nextChar == 0)
return;
out.write(cb, 0, nextChar);
nextChar = 0;
}
    }

/**
     * Writes a single character.
     *
     * @exception  IOException  If an I/O error occurs
     */
//写单个字符
public void write(int c) throws IOException {
synchronized (lock) {
            ensureOpen();
            if (nextChar >= nChars)//当前字符位置大于等于字符总数时，刷新
flushBuffer();
cb[nextChar++] = (char) c;
}
    }

/**
     * Our own little min method, to avoid loading java.lang.Math if we've run
     * out of file descriptors and we're trying to print a stack trace.
     */
//获取最小值
private int min(int a, int b) {
if (a < b) return a;
        return b;
}

/**
     * Writes a portion of an array of characters.
     *
     * <p> Ordinarily this method stores characters from the given array into
     * this stream's buffer, flushing the buffer to the underlying stream as
     * needed.  If the requested length is at least as large as the buffer,
     * however, then this method will flush the buffer and write the characters
     * directly to the underlying stream.  Thus redundant
     * <code>BufferedWriter</code>s will not copy data unnecessarily.
     *
     * @param  cbuf  A character array
     * @param  off   Offset from which to start reading characters
     * @param  len   Number of characters to write
     *
     * @exception  IOException  If an I/O error occurs
     */
//写cbuf的起始off,长度len
public void write(char cbuf[], int off, int len) throws IOException {
synchronized (lock) {
            ensureOpen();
            if ((off < 0) || (off > cbuf.length) || (len < 0) ||
                    ((off + len) > cbuf.length) || ((off + len) < 0)) {
throw new IndexOutOfBoundsException();
} else if (len == 0) {
return;
}

if (len >= nChars) {//如果len已经大于nChar,不再放到缓冲区，直接写
/* If the request length exceeds the size of the output buffer,
                   flush the buffer and then write the data directly.  In this
                   way buffered streams will cascade harmlessly. */
flushBuffer();
out.write(cbuf, off, len);
                return;
}

int b = off, t = off + len;
            while (b < t) {
int d = min(nChars - nextChar, t - b);
System.arraycopy(cbuf, b, cb, nextChar, d);
b += d;
nextChar += d;
                if (nextChar >= nChars)
                    flushBuffer();
}
        }
    }

/**
     * Writes a portion of a String.
     *
     * <p> If the value of the <tt>len</tt> parameter is negative then no
     * characters are written.  This is contrary to the specification of this
     * method in the {@linkplain java.io.Writer#write(java.lang.String,int,int)
     * superclass}, which requires that an {@link IndexOutOfBoundsException} be
     * thrown.
     *
     * @param  s     String to be written
     * @param  off   Offset from which to start reading characters
     * @param  len   Number of characters to be written
     *
     * @exception  IOException  If an I/O error occurs
     */
//写字符串
public void write(String s, int off, int len) throws IOException {
synchronized (lock) {
            ensureOpen();

            int b = off, t = off + len;
            while (b < t) {
int d = min(nChars - nextChar, t - b);
s.getChars(b, b + d, cb, nextChar);
b += d;
nextChar += d;
                if (nextChar >= nChars)
                    flushBuffer();
}
        }
    }

/**
     * Writes a line separator.  The line separator string is defined by the
     * system property <tt>line.separator</tt>, and is not necessarily a single
     * newline ('\n') character.
     *
     * @exception  IOException  If an I/O error occurs
     */
public void newLine() throws IOException {
        write(lineSeparator);
}

/**
     * Flushes the stream.
     *
     * @exception  IOException  If an I/O error occurs
     */
public void flush() throws IOException {
synchronized (lock) {
            flushBuffer();
out.flush();
}
    }

@SuppressWarnings("try")
public void close() throws IOException {
synchronized (lock) {
if (out == null) {
return;
}
try (Writer w = out) {
                flushBuffer();
} finally {
out = null;
cb = null;
}
        }
    }
}