Java 日看一类（5）之IO包中的BufferedWriter类

引入包和继承关系：

无引入包

继承自Writer类（类似于BufferReader）

代码注释：

/**
 * Writes text to a character-output stream, buffering characters so as to
 * provide for the efficient writing of single characters, arrays, and strings.
 *
 * <p> The buffer size may be specified, or the default size may be accepted.
 * The default is large enough for most purposes.
 *
 * <p> A newLine() method is provided, which uses the platform's own notion of
 * line separator as defined by the system property <tt>line.separator</tt>.
 * Not all platforms use the newline character ('\n') to terminate lines.
 * Calling this method to terminate each output line is therefore preferred to
 * writing a newline character directly.
 *
 * <p> In general, a Writer sends its output immediately to the underlying
 * character or byte stream.  Unless prompt output is required, it is advisable
 * to wrap a BufferedWriter around any Writer whose write() operations may be
 * costly, such as FileWriters and OutputStreamWriters.  For example,
 *
 * <pre>
 * PrintWriter out
 *   = new PrintWriter(new BufferedWriter(new FileWriter("foo.out")));
 * </pre>
 *
 * will buffer the PrintWriter's output to the file.  Without buffering, each
 * invocation of a print() method would cause characters to be converted into
 * bytes that would then be written immediately to the file, which can be very
 * inefficient.
 *
 * @see PrintWriter
 * @see FileWriter
 * @see OutputStreamWriter
 * @see java.nio.file.Files#newBufferedWriter
 *
 * @author      Mark Reinhold
 * @since       JDK1.1
 */

大意如下：

向字符输出流写入文本时，对字符进行缓冲处理是一种高效的处理方式，尤其是逐个的字符写入，数组结构写入或者字符串结构写入

缓冲区的大小可被设定，默认大小足以满足一般情况的需求

提供了newLine（）方法，提供了基于不同平台的行分割方法，因为不是所有平台都是用'\n'作为换行符，所以使用newLine方法是首选。

一般情况下，Writer类总是立即将数据输出到底层的字符流或比特流中。除非程序需要即刻输出，否则使用BufferedWriter封装时最好的选择。（对于write（）方法开销较大的Writer类十分必要）

例如：

  PrintWriter out  = new PrintWriter(new BufferedWriter(new FileWriter("foo.out")));

对PrintWriter类进行了缓冲。如果没有缓冲，任意一个对print方法的调用将会使字符转化为byte并且立刻写入到文件中，这是非常低效率的。

BufferedWriter含有五个成员变量：

被其封装的Writer类：

private Writer out;

缓冲区：

private char cb[];

缓冲区最大长度（这个和前几个不一样）和当前读取位置指针：

private int nChars, nextChar;

默认缓冲区长度：

private static int defaultCharBufferSize = 8192;

行分割符字符串

private String lineSeparator;

含有十一个方法：

构造函数：（默认大小缓冲区）

public BufferedWriter(Writer out) {
    this(out, defaultCharBufferSize);
}

构造函数：（自定义大小）

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(//对操作权限不进行检查
        new sun.security.action.GetPropertyAction("line.separator"));//该类实现一个接口，对传入的字符串调用System.getProperty(),获取当前系统行分割符（line.separator为该方法的保留限定字）
}

检测输出流状态：

private void ensureOpen() throws IOException {
    if (out == null)
        throw new IOException("Stream closed");
}

清空缓冲区（将缓冲区内容全部写入输出流）：

void flushBuffer() throws IOException {
    synchronized (lock) {
        ensureOpen();
        if (nextChar == 0)
            return;
        out.write(cb, 0, nextChar);
        nextChar = 0;
    }
}

向缓冲区中写入字符：

public void write(int c) throws IOException {
    synchronized (lock) {
        ensureOpen();
        if (nextChar >= nChars)//缓冲区满了就清空缓冲区
            flushBuffer();
        cb[nextChar++] = (char) c;
    }
}

两数求最小值：（我也不知道源码里为啥有个这……和前面几个风格完全不搭）

private int min(int a, int b) {
    if (a < b) return a;
    return b;
}

向缓冲区中写入数组：

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)) {//判定数组有效性
            throw new IndexOutOfBoundsException();
        } else if (len == 0) {
            return;
        }

        if (len >= nChars) {//写入长度大于缓冲区长度（前几个里面nChars都是标记缓冲区当前有效长度的，这个突然变成了最大长度……应该是换人写了）
            /* 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) {//循环写入
            int d = min(nChars - nextChar, t - b);
            System.arraycopy(cbuf, b, cb, nextChar, d);
            b += d;
            nextChar += d;
            if (nextChar >= nChars)
                flushBuffer();
        }
    }
}

写入字符串：

public void write(String s, int off, int len) throws IOException {
    synchronized (lock) {
        ensureOpen();

        int b = off, t = off + len;
        while (b < t) {//循环写入
            int d = min(nChars - nextChar, t - b);
            s.getChars(b, b + d, cb, nextChar);//字符串转字符
            b += d;
            nextChar += d;
            if (nextChar >= nChars)
                flushBuffer();
        }
    }
}

换行（在数据中插入换行符）：

public void newLine() throws IOException {
    write(lineSeparator);
}

将数据全部输出：

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

关闭流：

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

和前几个做的Buffered类的代码命名习惯有显著差别，不过整体思路没有大变化