Class - java.io.BufferedReader
Created by : Mr Dk.
2020 / 10 / 11 15:41
Nanjing, Jiangsu, China
Definition
与 java.io.BufferedInputStream 功能类似，将输入来源中的数据存放在一个 buffer 中。当 buffer 中的数据被读完时，再从输入来源中继续读取。区别在于 buffer 的数据类型不同，一个是 byte[]，一个是 char[]。这个类的用于主要在于提升性能，推荐作为一些慢速 Reader 的外层封装，比如可以封装在 FileReader 外面：
BufferedReader in = new BufferedReader(new FileReader("foo.in"));
这样，调用 read() 时，BufferedReader 会一次读取大量数据到 buffer 中，性能上会比 FileReader 每次读取一小部分要更好 (多次 I/O)。Buffer 的大小用户可以自行设置。
/**
* Reads text from a character-input stream, buffering characters so as to
* provide for the efficient reading of characters, arrays, and lines.
*
* <p> The buffer size may be specified, or the default size may be used. The
* default is large enough for most purposes.
*
* <p> In general, each read request made of a Reader causes a corresponding
* read request to be made of the underlying character or byte stream. It is
* therefore advisable to wrap a BufferedReader around any Reader whose read()
* operations may be costly, such as FileReaders and InputStreamReaders. For
* example,
*
* <pre>
* BufferedReader in
* = new BufferedReader(new FileReader("foo.in"));
* </pre>
*
* will buffer the input from the specified file. Without buffering, each
* invocation of read() or readLine() could cause bytes to be read from the
* file, converted into characters, and then returned, which can be very
* inefficient.
*
* <p> Programs that use DataInputStreams for textual input can be localized by
* replacing each DataInputStream with an appropriate BufferedReader.
*
* @see FileReader
* @see InputStreamReader
* @see java.nio.file.Files#newBufferedReader
*
* @author Mark Reinhold
* @since JDK1.1
*/
public class BufferedReader extends Reader {
}
其中，与上述特性相关的内部成员变量：
in负责维护输入来源 (底层是 InputStream)cb数组就是 buffer
private Reader in;
private char cb[];
以下成员变量用于处理不同操作系统平台对于换行符处理的差异：
/** If the next character is a line feed, skip it */
private boolean skipLF = false;
Constructor
构造函数的目的显而易见：
- 将输入来源维护在对象内
- 初始化 buffer 数组至指定长度 (如果用户未指定，则使用默认长度 8192)
private static int defaultCharBufferSize = 8192;
/**
* Creates a buffering character-input stream that uses an input buffer of
* the specified size.
*
* @param in A Reader
* @param sz Input-buffer size
*
* @exception IllegalArgumentException If {@code sz <= 0}
*/
public BufferedReader(Reader in, int sz) {
super(in);
if (sz <= 0)
throw new IllegalArgumentException("Buffer size <= 0");
this.in = in;
cb = new char[sz];
nextChar = nChars = 0;
}
/**
* Creates a buffering character-input stream that uses a default-sized
* input buffer.
*
* @param in A Reader
*/
public BufferedReader(Reader in) {
this(in, defaultCharBufferSize);
}
Fill
这个函数用于填充 buffer，当 buffer 中的字符已经不够读取时被调用。填充 buffer 的方法显而易见：从输入来源中调用 read() 即可。当然，需要根据当前对象是否启用了 mark 功能，以及 buffer 中的剩余空间，来决定从输入来源中读取的字节数。
其中，dst 变量指示了从输入流中读取字符后，存放到 buffer 中的位置；nextChar 指示下一个从当前 Reader 中被读取出的字符，nChars 指代 buffer 中最后一个有效字节的位置。如果当前 Reader 没有启用 mark，那么 buffer 中原有的内容已经全部被读取完毕，新读入的字符可以直接从 buffer 的头部开始存放，因此 dst 是 0。否则，至少需要保留从 markedChar 开始的字符。
如果说需要保留的字符已经超出了 readAheadLimit，那么 mark 就相当于作废了，也就是说 buffer 已经无法做到在保留原有 mark 字符的同时读取新的字符。如果没有超出 readAheadLimit，那么再判断这个值有没有超出 buffer 的长度：
- 如果没有超出，那么将 mark 位置开始的所有字符搬运到 buffer 开头，相当于丢弃 mark 位置之前的所有字符
- 如果超出，那么重新分配 buffer 数组，使其长度符合
readAheadLimit，然后再将 mark 位置开始的所有字符搬运到 buffer 开头
把原有的字符搬到 buffer 开头后，之后空出的位置就可以用于存放调用 read() 从输入流中获得的字符。
private static final int INVALIDATED = -2;
private static final int UNMARKED = -1;
private int markedChar = UNMARKED;
private int readAheadLimit = 0; /* Valid only when markedChar > 0 */
/**
* Fills the input buffer, taking the mark into account if it is valid.
*/
private void fill() throws IOException {
int dst;
if (markedChar <= UNMARKED) {
/* No mark */
dst = 0;
} else {
/* Marked */
int delta = nextChar - markedChar;
if (delta >= readAheadLimit) {
/* Gone past read-ahead limit: Invalidate mark */
markedChar = INVALIDATED;
readAheadLimit = 0;
dst = 0;
} else {
if (readAheadLimit <= cb.length) {
/* Shuffle in the current buffer */
System.arraycopy(cb, markedChar, cb, 0, delta);
markedChar = 0;
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 {
n = in.read(cb, dst, cb.length - dst);
} while (n == 0);
if (n > 0) {
nChars = dst + n;
nextChar = dst;
}
}
Read
以下函数从当前 Reader 中读取一个字符。在确保输入流不为空的前提下，返回 buffer 中的下一个字符。如果下一个字符已经超出了 buffer 的维护范围，那么调用一次 fill() 填充 buffer：
/**
* Reads a single character.
*
* @return The character read, as an integer in the range
* 0 to 65535 (<tt>0x00-0xffff</tt>), or -1 if the
* end of the stream has been reached
* @exception IOException If an I/O error occurs
*/
public int read() throws IOException {
synchronized (lock) {
ensureOpen();
for (;;) {
if (nextChar >= nChars) {
fill();
if (nextChar >= nChars)
return -1;
}
if (skipLF) {
skipLF = false;
if (cb[nextChar] == '\n') {
nextChar++;
continue;
}
}
return cb[nextChar++];
}
}
}
以下函数读取一段字符到指定数组内，是内部函数。如果读取的长度超出了 buffer 的长度且没有 mark/reset，那么直接调用底层 Reader 的 read()。如果剩余要读的字符不够了，则调用一次 fill() 填充 buffer。然后将 buffer 中的指定个数字符复制到目标数组中，并返回实际复制的字符数：
/**
* Reads characters into a portion of an array, reading from the underlying
* stream if necessary.
*/
private int read1(char[] cbuf, int off, int len) throws IOException {
if (nextChar >= nChars) {
/* If the requested length is at least as large as the buffer, and
if there is no mark/reset activity, and if line feeds are not
being skipped, do not bother to copy the characters into the
local buffer. In this way buffered streams will cascade
harmlessly. */
if (len >= cb.length && markedChar <= UNMARKED && !skipLF) {
return in.read(cbuf, off, len);
}
fill();
}
if (nextChar >= nChars) return -1;
if (skipLF) {
skipLF = false;
if (cb[nextChar] == '\n') {
nextChar++;
if (nextChar >= nChars)
fill();
if (nextChar >= nChars)
return -1;
}
}
int n = Math.min(len, nChars - nextChar);
System.arraycopy(cb, nextChar, cbuf, off, n);
nextChar += n;
return n;
}
以下函数是暴露给用户的外部函数，基本功能与上述内部函数一致，但是加入了同步操作，在保证输入流开启的前提下，循环调用上述内部函数，尽可能读取到用户指定的字符长度并返回用户。
/**
* Reads characters into a portion of an array.
*
* <p> This method implements the general contract of the corresponding
* <code>{@link Reader#read(char[], int, int) read}</code> method of the
* <code>{@link Reader}</code> class. As an additional convenience, it
* attempts to read as many characters as possible by repeatedly invoking
* the <code>read</code> method of the underlying stream. This iterated
* <code>read</code> continues until one of the following conditions becomes
* true: <ul>
*
* <li> The specified number of characters have been read,
*
* <li> The <code>read</code> method of the underlying stream returns
* <code>-1</code>, indicating end-of-file, or
*
* <li> The <code>ready</code> method of the underlying stream
* returns <code>false</code>, indicating that further input requests
* would block.
*
* </ul> If the first <code>read</code> on the underlying stream returns
* <code>-1</code> to indicate end-of-file then this method returns
* <code>-1</code>. Otherwise this method returns the number of characters
* actually read.
*
* <p> Subclasses of this class are encouraged, but not required, to
* attempt to read as many characters as possible in the same fashion.
*
* <p> Ordinarily this method takes characters from this stream's character
* buffer, filling it from the underlying stream as necessary. If,
* however, the buffer is empty, the mark is not valid, and the requested
* length is at least as large as the buffer, then this method will read
* characters directly from the underlying stream into the given array.
* Thus redundant <code>BufferedReader</code>s will not copy data
* unnecessarily.
*
* @param cbuf Destination buffer
* @param off Offset at which to start storing characters
* @param len Maximum number of characters to read
*
* @return The number of characters read, or -1 if the end of the
* stream has been reached
*
* @exception IOException If an I/O error occurs
*/
public int read(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 0;
}
int n = read1(cbuf, off, len);
if (n <= 0) return n;
while ((n < len) && in.ready()) {
int n1 = read1(cbuf, off + n, len - n);
if (n1 <= 0) break;
n += n1;
}
return n;
}
}
以下函数返回一行字符串。该函数将 \n (Line Feed, LF) (Unix)、\r (Carriage Return, CR, 显示为 ^M) (OSX)、\r\n (CRLF) (Windows) 中的任意一个视为行分隔符。区别。函数参数中的 boolean 变量决定了在返回的字符串中是否忽略最后的换行符，默认保留换行符。
内部实现会建立一个默认的 StringBuffer，默认一行的长度为 defaultExpectedLineLength (80)，然后在一个循环中不断读取字符，并 append 到 StringBuffer 后，直到识别出换行符为止。
private static int defaultExpectedLineLength = 80;
/**
* Reads a line of text. A line is considered to be terminated by any one
* of a line feed ('\n'), a carriage return ('\r'), or a carriage return
* followed immediately by a linefeed.
*
* @param ignoreLF If true, the next '\n' will be skipped
*
* @return A String containing the contents of the line, not including
* any line-termination characters, or null if the end of the
* stream has been reached
*
* @see java.io.LineNumberReader#readLine()
*
* @exception IOException If an I/O error occurs
*/
String readLine(boolean ignoreLF) throws IOException {
StringBuffer s = null;
int startChar;
synchronized (lock) {
ensureOpen();
boolean omitLF = ignoreLF || skipLF;
bufferLoop:
for (;;) {
if (nextChar >= nChars)
fill();
if (nextChar >= nChars) { /* EOF */
if (s != null && s.length() > 0)
return s.toString();
else
return null;
}
boolean eol = false;
char c = 0;
int i;
/* Skip a leftover '\n', if necessary */
if (omitLF && (cb[nextChar] == '\n'))
nextChar++;
skipLF = false;
omitLF = false;
charLoop:
for (i = nextChar; i < nChars; i++) {
c = cb[i];
if ((c == '\n') || (c == '\r')) {
eol = true;
break charLoop;
}
}
startChar = nextChar;
nextChar = i;
if (eol) {
String str;
if (s == null) {
str = new String(cb, startChar, i - startChar);
} else {
s.append(cb, startChar, i - startChar);
str = s.toString();
}
nextChar++;
if (c == '\r') {
skipLF = true;
}
return str;
}
if (s == null)
s = new StringBuffer(defaultExpectedLineLength);
s.append(cb, startChar, i - startChar);
}
}
}
/**
* Reads a line of text. A line is considered to be terminated by any one
* of a line feed ('\n'), a carriage return ('\r'), or a carriage return
* followed immediately by a linefeed.
*
* @return A String containing the contents of the line, not including
* any line-termination characters, or null if the end of the
* stream has been reached
*
* @exception IOException If an I/O error occurs
*
* @see java.nio.file.Files#readAllLines
*/
public String readLine() throws IOException {
return readLine(false);
}
以下函数返回一个可以被迭代器遍历的流，其中的内容就是每一行字符串：
/**
* Returns a {@code Stream}, the elements of which are lines read from
* this {@code BufferedReader}. The {@link Stream} is lazily populated,
* i.e., read only occurs during the
* <a href="../util/stream/package-summary.html#StreamOps">terminal
* stream operation</a>.
*
* <p> The reader must not be operated on during the execution of the
* terminal stream operation. Otherwise, the result of the terminal stream
* operation is undefined.
*
* <p> After execution of the terminal stream operation there are no
* guarantees that the reader will be at a specific position from which to
* read the next character or line.
*
* <p> If an {@link IOException} is thrown when accessing the underlying
* {@code BufferedReader}, it is wrapped in an {@link
* UncheckedIOException} which will be thrown from the {@code Stream}
* method that caused the read to take place. This method will return a
* Stream if invoked on a BufferedReader that is closed. Any operation on
* that stream that requires reading from the BufferedReader after it is
* closed, will cause an UncheckedIOException to be thrown.
*
* @return a {@code Stream<String>} providing the lines of text
* described by this {@code BufferedReader}
*
* @since 1.8
*/
public Stream<String> lines() {
Iterator<String> iter = new Iterator<String>() {
String nextLine = null;
@Override
public boolean hasNext() {
if (nextLine != null) {
return true;
} else {
try {
nextLine = readLine();
return (nextLine != null);
} catch (IOException e) {
throw new UncheckedIOException(e);
}
}
}
@Override
public String next() {
if (nextLine != null || hasNext()) {
String line = nextLine;
nextLine = null;
return line;
} else {
throw new NoSuchElementException();
}
}
};
return StreamSupport.stream(Spliterators.spliteratorUnknownSize(
iter, Spliterator.ORDERED | Spliterator.NONNULL), false);
}
Skip
跳过一些字节。当 buffer 中的有效字节用尽时，调用一次 fill()。
/**
* Skips characters.
*
* @param n The number of characters to skip
*
* @return The number of characters actually skipped
*
* @exception IllegalArgumentException If <code>n</code> is negative.
* @exception IOException If an I/O error occurs
*/
public long skip(long n) throws IOException {
if (n < 0L) {
throw new IllegalArgumentException("skip value is negative");
}
synchronized (lock) {
ensureOpen();
long r = n;
while (r > 0) {
if (nextChar >= nChars)
fill();
if (nextChar >= nChars) /* EOF */
break;
if (skipLF) {
skipLF = false;
if (cb[nextChar] == '\n') {
nextChar++;
}
}
long d = nChars - nextChar;
if (r <= d) {
nextChar += r;
r = 0;
break;
}
else {
r -= d;
nextChar = nChars;
}
}
return n - r;
}
}
Ready
当 buffer 中还可以有字符被读取或底层的输入流准备就绪 (调用 read() 不会阻塞) 时，当前函数返回 true，表示当前 Reader 准备就绪。默认 LF 会被跳过，不会被当作新一行的字符，保证只有当下一行字符准备就绪时，当前函数才返回 true。
/**
* Tells whether this stream is ready to be read. A buffered character
* stream is ready if the buffer is not empty, or if the underlying
* character stream is ready.
*
* @exception IOException If an I/O error occurs
*/
public boolean ready() throws IOException {
synchronized (lock) {
ensureOpen();
/*
* If newline needs to be skipped and the next char to be read
* is a newline character, then just skip it right away.
*/
if (skipLF) {
/* Note that in.ready() will return true if and only if the next
* read on the stream will not block.
*/
if (nextChar >= nChars && in.ready()) {
fill();
}
if (nextChar < nChars) {
if (cb[nextChar] == '\n')
nextChar++;
skipLF = false;
}
}
return (nextChar < nChars) || in.ready();
}
}
Mark / Reset
这个类支持 mark / reset 操作。在调用 mark() 时，需要指定 readAheadLimit。当 mark 的位置距离当前 read 的位置超过这个阈值时，reset 将无法回到 mark 所在位置而失败 (IOException)。如果这个值设置为超过 buffer 长度时，buffer 将会被扩展到不小于这个长度。
/**
* Tells whether this stream supports the mark() operation, which it does.
*/
public boolean markSupported() {
return true;
}
/**
* Marks the present position in the stream. Subsequent calls to reset()
* will attempt to reposition the stream to this point.
*
* @param readAheadLimit Limit on the number of characters that may be
* read while still preserving the mark. An attempt
* to reset the stream after reading characters
* up to this limit or beyond may fail.
* A limit value larger than the size of the input
* buffer will cause a new buffer to be allocated
* whose size is no smaller than limit.
* Therefore large values should be used with care.
*
* @exception IllegalArgumentException If {@code readAheadLimit < 0}
* @exception IOException If an I/O error occurs
*/
public void mark(int readAheadLimit) throws IOException {
if (readAheadLimit < 0) {
throw new IllegalArgumentException("Read-ahead limit < 0");
}
synchronized (lock) {
ensureOpen();
this.readAheadLimit = readAheadLimit;
markedChar = nextChar;
markedSkipLF = skipLF;
}
}
/**
* Resets the stream to the most recent mark.
*
* @exception IOException If the stream has never been marked,
* or if the mark has been invalidated
*/
public void reset() throws IOException {
synchronized (lock) {
ensureOpen();
if (markedChar < 0)
throw new IOException((markedChar == INVALIDATED)
? "Mark invalid"
: "Stream not marked");
nextChar = markedChar;
skipLF = markedSkipLF;
}
}
Close
关闭底层的 Reader：
public void close() throws IOException {
synchronized (lock) {
if (in == null)
return;
try {
in.close();
} finally {
in = null;
cb = null;
}
}
}