Class - java.io.DataOutputStream
Created by : Mr Dk.
2020 / 12 / 05 16:55
Nanjing, Jiangsu, China
Definition
该类使得应用能够向输出流写入原生 Java 数据类型。其实现的 DataOutput 接口定义了重载的 write()，它们只有参数不同，用于向输出流写入不同的 Java 原生数据类型。
/**
* A data output stream lets an application write primitive Java data
* types to an output stream in a portable way. An application can
* then use a data input stream to read the data back in.
*
* @author unascribed
* @see java.io.DataInputStream
* @since JDK1.0
*/
public
class DataOutputStream extends FilterOutputStream implements DataOutput {
}
Constructor
构造函数非常简单，传入一个输出流即可。这个输出流会作为父类 FilterOutputStream 内部维护的那个输出流。
/**
* Creates a new data output stream to write data to the specified
* underlying output stream. The counter <code>written</code> is
* set to zero.
*
* @param out the underlying output stream, to be saved for later
* use.
* @see java.io.FilterOutputStream#out
*/
public DataOutputStream(OutputStream out) {
super(out);
}
Write
首先是提供两个最原始版本的 write() (也是 OutputStream 抽象类中定义的)。其中，该类内部维护了一个已经写入字节的计数器，因此除了调用底层输出流的 write() 外，还需要累加这个计数器。计数器的维护方式如下：
/**
* The number of bytes written to the data output stream so far.
* If this counter overflows, it will be wrapped to Integer.MAX_VALUE.
*/
protected int written;
/**
* Increases the written counter by the specified value
* until it reaches Integer.MAX_VALUE.
*/
private void incCount(int value) {
int temp = written + value;
if (temp < 0) {
temp = Integer.MAX_VALUE;
}
written = temp;
}
两个最原始的 write() 实现如下：
/**
* Writes the specified byte (the low eight bits of the argument
* <code>b</code>) to the underlying output stream. If no exception
* is thrown, the counter <code>written</code> is incremented by
* <code>1</code>.
* <p>
* Implements the <code>write</code> method of <code>OutputStream</code>.
*
* @param b the <code>byte</code> to be written.
* @exception IOException if an I/O error occurs.
* @see java.io.FilterOutputStream#out
*/
public synchronized void write(int b) throws IOException {
out.write(b);
incCount(1);
}
/**
* Writes <code>len</code> bytes from the specified byte array
* starting at offset <code>off</code> to the underlying output stream.
* If no exception is thrown, the counter <code>written</code> is
* incremented by <code>len</code>.
*
* @param b the data.
* @param off the start offset in the data.
* @param len the number of bytes to write.
* @exception IOException if an I/O error occurs.
* @see java.io.FilterOutputStream#out
*/
public synchronized void write(byte b[], int off, int len)
throws IOException
{
out.write(b, off, len);
incCount(len);
}
接下来是所有 Java 原生类型以及常用数据类型 (如字符串) 的 write() 变种：
/**
* Writes a <code>boolean</code> to the underlying output stream as
* a 1-byte value. The value <code>true</code> is written out as the
* value <code>(byte)1</code>; the value <code>false</code> is
* written out as the value <code>(byte)0</code>. If no exception is
* thrown, the counter <code>written</code> is incremented by
* <code>1</code>.
*
* @param v a <code>boolean</code> value to be written.
* @exception IOException if an I/O error occurs.
* @see java.io.FilterOutputStream#out
*/
public final void writeBoolean(boolean v) throws IOException {
out.write(v ? 1 : 0);
incCount(1);
}
/**
* Writes out a <code>byte</code> to the underlying output stream as
* a 1-byte value. If no exception is thrown, the counter
* <code>written</code> is incremented by <code>1</code>.
*
* @param v a <code>byte</code> value to be written.
* @exception IOException if an I/O error occurs.
* @see java.io.FilterOutputStream#out
*/
public final void writeByte(int v) throws IOException {
out.write(v);
incCount(1);
}
/**
* Writes a <code>short</code> to the underlying output stream as two
* bytes, high byte first. If no exception is thrown, the counter
* <code>written</code> is incremented by <code>2</code>.
*
* @param v a <code>short</code> to be written.
* @exception IOException if an I/O error occurs.
* @see java.io.FilterOutputStream#out
*/
public final void writeShort(int v) throws IOException {
out.write((v >>> 8) & 0xFF);
out.write((v >>> 0) & 0xFF);
incCount(2);
}
/**
* Writes a <code>char</code> to the underlying output stream as a
* 2-byte value, high byte first. If no exception is thrown, the
* counter <code>written</code> is incremented by <code>2</code>.
*
* @param v a <code>char</code> value to be written.
* @exception IOException if an I/O error occurs.
* @see java.io.FilterOutputStream#out
*/
public final void writeChar(int v) throws IOException {
out.write((v >>> 8) & 0xFF);
out.write((v >>> 0) & 0xFF);
incCount(2);
}
/**
* Writes an <code>int</code> to the underlying output stream as four
* bytes, high byte first. If no exception is thrown, the counter
* <code>written</code> is incremented by <code>4</code>.
*
* @param v an <code>int</code> to be written.
* @exception IOException if an I/O error occurs.
* @see java.io.FilterOutputStream#out
*/
public final void writeInt(int v) throws IOException {
out.write((v >>> 24) & 0xFF);
out.write((v >>> 16) & 0xFF);
out.write((v >>> 8) & 0xFF);
out.write((v >>> 0) & 0xFF);
incCount(4);
}
private byte writeBuffer[] = new byte[8];
/**
* Writes a <code>long</code> to the underlying output stream as eight
* bytes, high byte first. In no exception is thrown, the counter
* <code>written</code> is incremented by <code>8</code>.
*
* @param v a <code>long</code> to be written.
* @exception IOException if an I/O error occurs.
* @see java.io.FilterOutputStream#out
*/
public final void writeLong(long v) throws IOException {
writeBuffer[0] = (byte)(v >>> 56);
writeBuffer[1] = (byte)(v >>> 48);
writeBuffer[2] = (byte)(v >>> 40);
writeBuffer[3] = (byte)(v >>> 32);
writeBuffer[4] = (byte)(v >>> 24);
writeBuffer[5] = (byte)(v >>> 16);
writeBuffer[6] = (byte)(v >>> 8);
writeBuffer[7] = (byte)(v >>> 0);
out.write(writeBuffer, 0, 8);
incCount(8);
}
/**
* Converts the float argument to an <code>int</code> using the
* <code>floatToIntBits</code> method in class <code>Float</code>,
* and then writes that <code>int</code> value to the underlying
* output stream as a 4-byte quantity, high byte first. If no
* exception is thrown, the counter <code>written</code> is
* incremented by <code>4</code>.
*
* @param v a <code>float</code> value to be written.
* @exception IOException if an I/O error occurs.
* @see java.io.FilterOutputStream#out
* @see java.lang.Float#floatToIntBits(float)
*/
public final void writeFloat(float v) throws IOException {
writeInt(Float.floatToIntBits(v));
}
/**
* Converts the double argument to a <code>long</code> using the
* <code>doubleToLongBits</code> method in class <code>Double</code>,
* and then writes that <code>long</code> value to the underlying
* output stream as an 8-byte quantity, high byte first. If no
* exception is thrown, the counter <code>written</code> is
* incremented by <code>8</code>.
*
* @param v a <code>double</code> value to be written.
* @exception IOException if an I/O error occurs.
* @see java.io.FilterOutputStream#out
* @see java.lang.Double#doubleToLongBits(double)
*/
public final void writeDouble(double v) throws IOException {
writeLong(Double.doubleToLongBits(v));
}
/**
* Writes out the string to the underlying output stream as a
* sequence of bytes. Each character in the string is written out, in
* sequence, by discarding its high eight bits. If no exception is
* thrown, the counter <code>written</code> is incremented by the
* length of <code>s</code>.
*
* @param s a string of bytes to be written.
* @exception IOException if an I/O error occurs.
* @see java.io.FilterOutputStream#out
*/
public final void writeBytes(String s) throws IOException {
int len = s.length();
for (int i = 0 ; i < len ; i++) {
out.write((byte)s.charAt(i));
}
incCount(len);
}
/**
* Writes a string to the underlying output stream as a sequence of
* characters. Each character is written to the data output stream as
* if by the <code>writeChar</code> method. If no exception is
* thrown, the counter <code>written</code> is incremented by twice
* the length of <code>s</code>.
*
* @param s a <code>String</code> value to be written.
* @exception IOException if an I/O error occurs.
* @see java.io.DataOutputStream#writeChar(int)
* @see java.io.FilterOutputStream#out
*/
public final void writeChars(String s) throws IOException {
int len = s.length();
for (int i = 0 ; i < len ; i++) {
int v = s.charAt(i);
out.write((v >>> 8) & 0xFF);
out.write((v >>> 0) & 0xFF);
}
incCount(len * 2);
}
/**
* Writes a string to the underlying output stream using
* <a href="DataInput.html#modified-utf-8">modified UTF-8</a>
* encoding in a machine-independent manner.
* <p>
* First, two bytes are written to the output stream as if by the
* <code>writeShort</code> method giving the number of bytes to
* follow. This value is the number of bytes actually written out,
* not the length of the string. Following the length, each character
* of the string is output, in sequence, using the modified UTF-8 encoding
* for the character. If no exception is thrown, the counter
* <code>written</code> is incremented by the total number of
* bytes written to the output stream. This will be at least two
* plus the length of <code>str</code>, and at most two plus
* thrice the length of <code>str</code>.
*
* @param str a string to be written.
* @exception IOException if an I/O error occurs.
*/
public final void writeUTF(String str) throws IOException {
writeUTF(str, this);
}
/**
* Writes a string to the specified DataOutput using
* <a href="DataInput.html#modified-utf-8">modified UTF-8</a>
* encoding in a machine-independent manner.
* <p>
* First, two bytes are written to out as if by the <code>writeShort</code>
* method giving the number of bytes to follow. This value is the number of
* bytes actually written out, not the length of the string. Following the
* length, each character of the string is output, in sequence, using the
* modified UTF-8 encoding for the character. If no exception is thrown, the
* counter <code>written</code> is incremented by the total number of
* bytes written to the output stream. This will be at least two
* plus the length of <code>str</code>, and at most two plus
* thrice the length of <code>str</code>.
*
* @param str a string to be written.
* @param out destination to write to
* @return The number of bytes written out.
* @exception IOException if an I/O error occurs.
*/
static int writeUTF(String str, DataOutput out) throws IOException {
int strlen = str.length();
int utflen = 0;
int c, count = 0;
/* use charAt instead of copying String to char array */
for (int i = 0; i < strlen; i++) {
c = str.charAt(i);
if ((c >= 0x0001) && (c <= 0x007F)) {
utflen++;
} else if (c > 0x07FF) {
utflen += 3;
} else {
utflen += 2;
}
}
if (utflen > 65535)
throw new UTFDataFormatException(
"encoded string too long: " + utflen + " bytes");
byte[] bytearr = null;
if (out instanceof DataOutputStream) {
DataOutputStream dos = (DataOutputStream)out;
if(dos.bytearr == null || (dos.bytearr.length < (utflen+2)))
dos.bytearr = new byte[(utflen*2) + 2];
bytearr = dos.bytearr;
} else {
bytearr = new byte[utflen+2];
}
bytearr[count++] = (byte) ((utflen >>> 8) & 0xFF);
bytearr[count++] = (byte) ((utflen >>> 0) & 0xFF);
int i=0;
for (i=0; i<strlen; i++) {
c = str.charAt(i);
if (!((c >= 0x0001) && (c <= 0x007F))) break;
bytearr[count++] = (byte) c;
}
for (;i < strlen; i++){
c = str.charAt(i);
if ((c >= 0x0001) && (c <= 0x007F)) {
bytearr[count++] = (byte) c;
} else if (c > 0x07FF) {
bytearr[count++] = (byte) (0xE0 | ((c >> 12) & 0x0F));
bytearr[count++] = (byte) (0x80 | ((c >> 6) & 0x3F));
bytearr[count++] = (byte) (0x80 | ((c >> 0) & 0x3F));
} else {
bytearr[count++] = (byte) (0xC0 | ((c >> 6) & 0x1F));
bytearr[count++] = (byte) (0x80 | ((c >> 0) & 0x3F));
}
}
out.write(bytearr, 0, utflen+2);
return utflen + 2;
}
Flush
直接调用底层的 flush()。
/**
* Flushes this data output stream. This forces any buffered output
* bytes to be written out to the stream.
* <p>
* The <code>flush</code> method of <code>DataOutputStream</code>
* calls the <code>flush</code> method of its underlying output stream.
*
* @exception IOException if an I/O error occurs.
* @see java.io.FilterOutputStream#out
* @see java.io.OutputStream#flush()
*/
public void flush() throws IOException {
out.flush();
}
Written Size
返回当前已经写入输出流的字节数。
/**
* Returns the current value of the counter <code>written</code>,
* the number of bytes written to this data output stream so far.
* If the counter overflows, it will be wrapped to Integer.MAX_VALUE.
*
* @return the value of the <code>written</code> field.
* @see java.io.DataOutputStream#written
*/
public final int size() {
return written;
}