java-基础知识-IO
0、问题
- 什么是流？
- 字节和字符有什么区别？
- CharArrayReader/Writer 实现的逻辑？
- protected的使用范围。
- String 说是固定不变的，为何他的内容会发生变化？
- flush 与close有什么区别？
1、CharArrayReader
1.0、内部变量
/** The character buffer. */
protected char buf[];
/** The current buffer position. */
protected int pos;
/** The position of mark in buffer. */
protected int markedPos = 0;
/** The index of the end of this buffer. There is not valid * data at or beyond this index. */
protected int count;
1.1、构造函数
- 构造函数，方便初始化
/**
* Creates a CharArrayReader from the specified array of chars.
* @param buf Input buffer (not copied)
*/
public CharArrayReader(char buf[]) {
this.buf = buf;
this.pos = 0;
this.count = buf.length;
}
- 构造函数，多参
/**
* Creates a CharArrayReader from the specified array of chars.
*
* <p> The resulting reader will start reading at the given
* <tt>offset</tt>. The total number of <tt>char</tt> values that can be
* read from this reader will be either <tt>length</tt> or
* <tt>buf.length-offset</tt>, whichever is smaller.
*
* @throws IllegalArgumentException
* If <tt>offset</tt> is negative or greater than
* <tt>buf.length</tt>, or if <tt>length</tt> is negative, or if
* the sum of these two values is negative.
*
* @param buf Input buffer (not copied)
* @param offset Offset of the first char to read
* @param length Number of chars to read
*/
public CharArrayReader(char buf[], int offset, int length) {
if ((offset < 0) || (offset > buf.length) || (length < 0) ||
((offset + length) < 0)) {
throw new IllegalArgumentException();
}
this.buf = buf;
this.pos = offset;
this.count = Math.min(offset + length, buf.length);
this.markedPos = offset;
}
- 检查流是否中途关闭
/** Checks to make sure that the stream has not been closed */
private void ensureOpen() throws IOException {
if (buf == null)
throw new IOException("Stream closed");
}
1.1 、读取字符
- 读取一个字符
/**
* Reads a single character.
*
* @exception IOException If an I/O error occurs
*/
public int read() throws IOException {
synchronized (lock) {
ensureOpen();
if (pos >= count)
return -1;
else
return buf[pos++];
}
}
- 根据传参读取多个字符，返回读取的长度
/**
* Reads characters into a portion of an array.
* @param b Destination buffer
* @param off Offset at which to start storing characters
* @param len Maximum number of characters to read
* @return The actual 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 b[], int off, int len) throws IOException {
synchronized (lock) {
ensureOpen();
if ((off < 0) || (off > b.length) || (len < 0) ||
((off + len) > b.length) || ((off + len) < 0)) {
throw new IndexOutOfBoundsException();
} else if (len == 0) {
return 0;
}
if (pos >= count) {
return -1;
}
if (pos + len > count) {
len = count - pos;
}
if (len <= 0) {
return 0;
}
System.arraycopy(buf, pos, b, off, len);
pos += len;
return len;
}
}
- 忽略字符，返回被忽略的长度
/**
* Skips characters. Returns the number of characters that were skipped.
*
* <p>The <code>n</code> parameter may be negative, even though the
* <code>skip</code> method of the {@link Reader} superclass throws
* an exception in this case. If <code>n</code> is negative, then
* this method does nothing and returns <code>0</code>.
*
* @param n The number of characters to skip
* @return The number of characters actually skipped
* @exception IOException If the stream is closed, or an I/O error occurs
*/
public long skip(long n) throws IOException {
synchronized (lock) {
ensureOpen();
if (pos + n > count) {
n = count - pos;
}
if (n < 0) {
return 0;
}
pos += n;
return n;
}
}
- 告诉流是否已经准备读，当前字符数组是否准备好
/**
* Tells whether this stream is ready to be read. Character-array readers
* are always ready to be read.
*
* @exception IOException If an I/O error occurs
*/
public boolean ready() throws IOException {
synchronized (lock) {
ensureOpen();
return (count - pos) > 0;
}
}
1.3 标记相关
- 是否支持标记/
/**
* Tells whether this stream supports the mark() operation, which it does.
*/
public boolean markSupported() {
return true;
}
- 开始标记
/**
* Marks the present position in the stream. Subsequent calls to reset()
* will reposition the stream to this point.
*
* @param readAheadLimit Limit on the number of characters that may be
* read while still preserving the mark. Because
* the stream's input comes from a character array,
* there is no actual limit; hence this argument is
* ignored.
*
* @exception IOException If an I/O error occurs
*/
public void mark(int readAheadLimit) throws IOException {
synchronized (lock) {
ensureOpen();
markedPos = pos;
}
}
- 重新回到标记处,再读
/**
* Resets the stream to the most recent mark, or to the beginning if it has
* never been marked.
*
* @exception IOException If an I/O error occurs
*/
public void reset() throws IOException {
synchronized (lock) {
ensureOpen();
pos = markedPos;
}
}
- 释放资源
/**
* Closes the stream and releases any system resources associated with
* it. Once the stream has been closed, further read(), ready(),
* mark(), reset(), or skip() invocations will throw an IOException.
* Closing a previously closed stream has no effect.
*/
public void close() {
buf = null;
}
2. CharArrayWriter
2.1 变量初始化与构造函数
/**
* The buffer where data is stored.
*/
protected char buf[];
/**
* The number of chars in the buffer.
*/
protected int count;
/**
* Creates a new CharArrayWriter.
*/
public CharArrayWriter() {
this(32);
}
/**
* Creates a new CharArrayWriter with the specified initial size.
*
* @param initialSize an int specifying the initial buffer size.
* @exception IllegalArgumentException if initialSize is negative
*/
public CharArrayWriter(int initialSize) {
if (initialSize < 0) {
throw new IllegalArgumentException("Negative initial size: "
+ initialSize);
}
buf = new char[initialSize];
}
2.2 写字符
- 单参(字符数组)
/**
* Writes a character to the buffer.
*/
public void write(int c) {
synchronized (lock) {
int newcount = count + 1;
if (newcount > buf.length) {
//默认32，扩大数组范围
buf = Arrays.copyOf(buf, Math.max(buf.length << 1, newcount));
}
buf[count] = (char)c;
count = newcount;
}
}
- 多参（字符数组）
/**
* Writes characters to the buffer.
* @param c the data to be written
* @param off the start offset in the data
* @param len the number of chars that are written
*/
public void write(char c[], int off, int len) {
if ((off < 0) || (off > c.length) || (len < 0) ||
((off + len) > c.length) || ((off + len) < 0)) {
throw new IndexOutOfBoundsException();
} else if (len == 0) {
return;
}
synchronized (lock) {
int newcount = count + len;
if (newcount > buf.length) {
buf = Arrays.copyOf(buf, Math.max(buf.length << 1, newcount));
}
System.arraycopy(c, off, buf, count, len);
count = newcount;
}
}
- 多参（字符串）
/**
* Write a portion of a string to the buffer.
* @param str String to be written from
* @param off Offset from which to start reading characters
* @param len Number of characters to be written
*/
public void write(String str, int off, int len) {
synchronized (lock) {
int newcount = count + len;
if (newcount > buf.length) {
buf = Arrays.copyOf(buf, Math.max(buf.length << 1, newcount));
}
str.getChars(off, off + len, buf, count);
count = newcount;
}
}
2.3 字符追加
- 单参
/**
* Appends the specified character sequence to this writer.
*
* <p> An invocation of this method of the form <tt>out.append(csq)</tt>
* behaves in exactly the same way as the invocation
*
* <pre>
* out.write(csq.toString()) </pre>
*
* <p> Depending on the specification of <tt>toString</tt> for the
* character sequence <tt>csq</tt>, the entire sequence may not be
* appended. For instance, invoking the <tt>toString</tt> method of a
* character buffer will return a subsequence whose content depends upon
* the buffer's position and limit.
*
* @param csq
* The character sequence to append. If <tt>csq</tt> is
* <tt>null</tt>, then the four characters <tt>"null"</tt> are
* appended to this writer.
*
* @return This writer
*
* @since 1.5
*/
public CharArrayWriter append(CharSequence csq) {
String s = (csq == null ? "null" : csq.toString());
write(s, 0, s.length());
return this;
}
- 多参
/**
* Appends a subsequence of the specified character sequence to this writer.
*
* <p> An invocation of this method of the form <tt>out.append(csq, start,
* end)</tt> when <tt>csq</tt> is not <tt>null</tt>, behaves in
* exactly the same way as the invocation
*
* <pre>
* out.write(csq.subSequence(start, end).toString()) </pre>
*
* @param csq
* The character sequence from which a subsequence will be
* appended. If <tt>csq</tt> is <tt>null</tt>, then characters
* will be appended as if <tt>csq</tt> contained the four
* characters <tt>"null"</tt>.
*
* @param start
* The index of the first character in the subsequence
*
* @param end
* The index of the character following the last character in the
* subsequence
*
* @return This writer
*
* @throws IndexOutOfBoundsException
* If <tt>start</tt> or <tt>end</tt> are negative, <tt>start</tt>
* is greater than <tt>end</tt>, or <tt>end</tt> is greater than
* <tt>csq.length()</tt>
*
* @since 1.5
*/
public CharArrayWriter append(CharSequence csq, int start, int end) {
String s = (csq == null ? "null" : csq).subSequence(start, end).toString();
write(s, 0, s.length());
return this;
}
- 追加字符
/**
* Appends the specified character to this writer.
*
* <p> An invocation of this method of the form <tt>out.append(c)</tt>
* behaves in exactly the same way as the invocation
*
* <pre>
* out.write(c) </pre>
*
* @param c
* The 16-bit character to append
*
* @return This writer
*
* @since 1.5
*/
public CharArrayWriter append(char c) {
write(c);
return this;
}
2.4 重置缓重，再次写入
/**
* Resets the buffer so that you can use it again without
* throwing away the already allocated buffer.
*/
public void reset() {
count = 0;
}
返回字符数组
/**
* Returns a copy of the input data.
*
* @return an array of chars copied from the input data.
*/
public char toCharArray()[] {
synchronized (lock) {
return Arrays.copyOf(buf, count);
}
}
2.5 返回当前缓冲区大小，冲刷缓冲区，释放资源
/**
* Returns the current size of the buffer.
*
* @return an int representing the current size of the buffer.
*/
public int size() {
return count;
}
/**
* Converts input data to a string.
* @return the string.
*/
public String toString() {
synchronized (lock) {
return new String(buf, 0, count);
}
}
/**
* Flush the stream.
*/
public void flush() { }
/**
* Close the stream. This method does not release the buffer, since its
* contents might still be required. Note: Invoking this method in this class
* will have no effect.
*/
public void close() { }
浙公网安备 33010602011771号