Admin: 1 версия

2010-06-01T05:20:21Z

1 версия

в 17:44, 31 мая 2010

2010-05-31T17:44:27Z

Новая страница

== A ByteBuffer is a fixed-capacity buffer that holds byte values. ==



<source lang="java">
import java.nio.ByteBuffer;
public class Main {
public static void main(String[] argv) throws Exception {

byte[] bytes = new byte[10];
ByteBuffer buf = ByteBuffer.wrap(bytes);
}
}</source>



== Allocation automatically zeroes the ByteBuffer ==



<source lang="java">
import java.nio.ByteBuffer;
public class MainClass {
private static final int BSIZE = 1024;
public static void main(String[] args) {
ByteBuffer bb = ByteBuffer.allocate(BSIZE);
int i = 0;
while (i++ < bb.limit())
if (bb.get() != 0)
System.out.println("nonzero");
System.out.println("i = " + i);
}
}</source>



== ByteBuffer.hasRemaining() ==



<source lang="java">
import java.nio.ByteBuffer;
public class MainClass {
public static void main(String[] args) {
ByteBuffer bb = ByteBuffer.wrap(new byte[] { 0, 0, 0, 0, 0, 0, 0, "a" });
bb.rewind();
System.out.println("Byte Buffer");
while (bb.hasRemaining())
System.out.println(bb.position() + " -> " + bb.get());
}
}
/**/</source>



<pre class=codeResult>Byte Buffer
0 -> 0
1 -> 0
2 -> 0
3 -> 0
4 -> 0
5 -> 0
6 -> 0
7 -> 97</pre>

== Converting Between a ByteBuffer an a Byte Array ==



<source lang="java">
import java.nio.ByteBuffer;
public class Main {
public static void main(String[] argv) throws Exception {
// Create a ByteBuffer from a byte array
byte[] bytes = new byte[10];
ByteBuffer buf = ByteBuffer.wrap(bytes);
}
}</source>



== Converting text to and from ByteBuffers ==



<source lang="java">
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.charset.Charset;
public class MainClass {
private static final int BSIZE = 1024;
public static void main(String[] args) throws Exception {
FileChannel fc = new FileOutputStream("data2.txt").getChannel();
fc.write(ByteBuffer.wrap("Some text".getBytes()));
fc.close();
fc = new FileInputStream("data2.txt").getChannel();
ByteBuffer buff = ByteBuffer.allocate(BSIZE);
fc.read(buff);
buff.flip();
System.out.println(buff.asCharBuffer());
// Decode using this system"s default Charset:
buff.rewind();
String encoding = System.getProperty("file.encoding");
System.out.println("Decoded using " + encoding + ": " + Charset.forName(encoding).decode(buff));
// Or, we could encode with something that will print:
fc = new FileOutputStream("data2.txt").getChannel();
fc.write(ByteBuffer.wrap("Some text".getBytes("UTF-16BE")));
fc.close();
}
}
/**/</source>



<pre class=codeResult>????
Decoded using Cp1252: Some text</pre>

== Converting text to and from ByteBuffers with UTF-16BE ==



<source lang="java">
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.charset.Charset;
public class MainClass {
private static final int BSIZE = 1024;
public static void main(String[] args) throws Exception {
FileChannel fc = new FileOutputStream("data2.txt").getChannel();
fc.write(ByteBuffer.wrap(
"Some text".getBytes("UTF-16BE")));
fc.close();
ByteBuffer buff = ByteBuffer.allocate(BSIZE);
// Now try reading again:
fc = new FileInputStream("data2.txt").getChannel();
buff.clear();
fc.read(buff);
buff.flip();
System.out.println(buff.asCharBuffer());

}
}
/**/</source>



<pre class=codeResult>Some text</pre>

== Create a ByteBuffer using a byte array ==



<source lang="java">
import java.nio.ByteBuffer;
public class Main {
public static void main(String[] argv) throws Exception {

byte[] bytes = new byte[10];
ByteBuffer buf = ByteBuffer.wrap(bytes);
}
}</source>



== Create a direct (memory-mapped) ByteBuffer with a 10 byte capacity. ==



<source lang="java">
import java.nio.ByteBuffer;
public class Main {
public static void main(String[] argv) throws Exception {
ByteBuffer buf = ByteBuffer.allocateDirect(10);
}
}</source>



== Create a non-direct ByteBuffer with a 10 byte capacity ==



<source lang="java">
import java.nio.ByteBuffer;
public class Main {
public static void main(String[] argv) throws Exception {
ByteBuffer buf = ByteBuffer.allocate(10);
}
}</source>



== Endian differences and data storage ==



<source lang="java">
import java.nio.ByteBuffer;
import java.nio.ByteOrder;
public class MainClass {
public static void main(String[] args) {
ByteBuffer bb = ByteBuffer.wrap(new byte[12]);
bb.asCharBuffer().put("abcdef");
System.out.println(toString(bb.array()));
bb.rewind();
bb.order(ByteOrder.BIG_ENDIAN);
bb.asCharBuffer().put("abcdef");
System.out.println(toString(bb.array()));
bb.rewind();
bb.order(ByteOrder.LITTLE_ENDIAN);
bb.asCharBuffer().put("abcdef");
System.out.println(toString(bb.array()));
}
public static String toString(byte[] a) {
StringBuffer result = new StringBuffer("[");
for (int i = 0; i < a.length; i++) {
result.append(a[i]);
if (i < a.length - 1)
result.append(", ");
}
result.append("]");
return result.toString();
}
}
/*
* [0, 97, 0, 98, 0, 99, 0, 100, 0, 101, 0, 102] [0, 97, 0, 98, 0, 99, 0, 100,
* 0, 101, 0, 102] [97, 0, 98, 0, 99, 0, 100, 0, 101, 0, 102, 0]
*/</source>



== Get remaining byte count in a ByteBuffer ==



<source lang="java">
import java.nio.ByteBuffer;
public class Main {
public static void main(String[] argv) throws Exception {
ByteBuffer buf = ByteBuffer.allocateDirect(10);
int rem = buf.remaining();
}
}</source>



== Get the ByteBuffer"s capacity ==



<source lang="java">
import java.nio.ByteBuffer;
public class Main {
public static void main(String[] argv) throws Exception {
ByteBuffer buf = ByteBuffer.allocateDirect(10);
int capacity = buf.capacity(); // 10
}
}</source>



== Putting Bytes into a ByteBuffer ==



<source lang="java">
import java.nio.ByteBuffer;
public class Main {
public static void main(String[] argv) throws Exception {
ByteBuffer bbuf = ByteBuffer.allocate(10);
int capacity = bbuf.capacity(); // 10
System.out.println(capacity);
bbuf.put((byte) 0xFF);
bbuf.position(5);
bbuf.put((byte) 0xFF);
int pos = bbuf.position();
int rem = bbuf.remaining();
bbuf.limit(7);
bbuf.rewind();
}
}</source>



== Read from a channel with a ByteBuffer ==



<source lang="java">
import java.io.FileInputStream;
import java.nio.ByteBuffer;
import java.nio.channels.ReadableByteChannel;
public class Main {
public static void main(String[] argv) throws Exception {
ReadableByteChannel channel = new FileInputStream("infile").getChannel();
ByteBuffer buf = ByteBuffer.allocateDirect(10);
int numRead = 0;
while (numRead >= 0) {
buf.rewind();
numRead = channel.read(buf);
buf.rewind();
for (int i = 0; i < numRead; i++) {
byte b = buf.get();
}
}
}
}</source>



== Retrieve all bytes in the buffer ==



<source lang="java">
import java.nio.ByteBuffer;
public class Main {
public static void main(String[] argv) throws Exception {
byte[] bytes = new byte[10];
ByteBuffer buf = ByteBuffer.wrap(bytes);
buf.clear();
bytes = new byte[buf.capacity()];
buf.get(bytes, 0, bytes.length);
}
}</source>



== Retrieve bytes between the position and limit ==



<source lang="java">
import java.nio.ByteBuffer;
public class Main {
public static void main(String[] argv) throws Exception {
byte[] bytes = new byte[10];
ByteBuffer buf = ByteBuffer.wrap(bytes);
bytes = new byte[buf.remaining()];
buf.get(bytes, 0, bytes.length);
}
}</source>



== Rewind a ByteBuffer ==



<source lang="java">
import java.nio.ByteBuffer;
public class MainClass {
private static final int BSIZE = 1024;
public static void main(String[] args) {
ByteBuffer bb = ByteBuffer.allocate(BSIZE);
bb.asCharBuffer().put("Howdy!");
char c;
while ((c = bb.getChar()) != 0)
System.out.print(c + " ");
System.out.println();
bb.rewind();
// Store and read a short:
bb.asShortBuffer().put((short) 471142);
System.out.println(bb.getShort());
}
}
/**/</source>



<pre class=codeResult>H o w d y !
12390</pre>

== Set the limit for ByteBuffer ==



<source lang="java">
import java.nio.ByteBuffer;
public class Main {
public static void main(String[] argv) throws Exception {
ByteBuffer buf = ByteBuffer.allocateDirect(10);
buf.limit(7); // remaining=1
}
}</source>



== Set the position ==



<source lang="java">
import java.nio.ByteBuffer;
public class Main {
public static void main(String[] argv) throws Exception {
ByteBuffer buf = ByteBuffer.allocateDirect(10);
buf.position(5);
}
}</source>



== Store and read a char array ==



<source lang="java">
import java.nio.ByteBuffer;
public class MainClass {
private static final int BSIZE = 1024;
public static void main(String[] args) {
ByteBuffer bb = ByteBuffer.allocate(BSIZE);
bb.asCharBuffer().put("Howdy!");
char c;
while ((c = bb.getChar()) != 0)
System.out.print(c + " ");
System.out.println();
}
}//</source>



<pre class=codeResult>H o w d y !</pre>

== Store and read a double ==



<source lang="java">
import java.nio.ByteBuffer;
public class MainClass {
private static final int BSIZE = 1024;
public static void main(String[] args) {
ByteBuffer bb = ByteBuffer.allocate(BSIZE);
bb.asDoubleBuffer().put(99471142);
System.out.println(bb.getDouble());
}
}
/*
*/</source>



<pre class=codeResult>9.9471142E7</pre>

== Store and read a float ==



<source lang="java">
import java.nio.ByteBuffer;
public class MainClass {
private static final int BSIZE = 1024;
public static void main(String[] args) {
ByteBuffer bb = ByteBuffer.allocate(BSIZE);
bb.asFloatBuffer().put(99471142);
System.out.println(bb.getFloat());
}
}
/**/</source>



<pre class=codeResult>9.9471144E7</pre>

== Store and read a long ==



<source lang="java">
import java.nio.ByteBuffer;
public class MainClass {
private static final int BSIZE = 1024;
public static void main(String[] args) {
ByteBuffer bb = ByteBuffer.allocate(BSIZE);
bb.asLongBuffer().put(99472342341142L);
System.out.println(bb.getLong());
}
}
/**/</source>



<pre class=codeResult>99472342341142</pre>

== Store and read an int ==



<source lang="java">
import java.nio.ByteBuffer;
public class MainClass {
private static final int BSIZE = 1024;
public static void main(String[] args) {
ByteBuffer bb = ByteBuffer.allocate(BSIZE);
bb.asIntBuffer().put(99471142);
System.out.println(bb.getInt());
}
}
/**/</source>



<pre class=codeResult>99471142</pre>

== Store and read a short ==



<source lang="java">
import java.nio.ByteBuffer;
public class MainClass {
private static final int BSIZE = 1024;
public static void main(String[] args) {
ByteBuffer bb = ByteBuffer.allocate(BSIZE);
bb.asCharBuffer().put("Howdy!");
char c;
while ((c = bb.getChar()) != 0)
System.out.print(c + " ");
System.out.println();
bb.rewind();
// Store and read a short:
bb.asShortBuffer().put((short) 471142);
System.out.println(bb.getShort());
}
}
/**/</source>



<pre class=codeResult>H o w d y !
12390</pre>

== Test views of long elements in a ByteBuffer ==



<source lang="java">
import java.nio.ByteBuffer;
import java.nio.ByteOrder;
/**
* Test views of long elements in a ByteBuffer.
*
*/
public class MainClass {
public static void main(String[] argv) throws Exception {
ByteBuffer bb = ByteBuffer.allocate(20);
bb.put((byte) 0x07);
bb.put((byte) 0x08);
bb.put((byte) 0x09);
bb.put((byte) 0x10);
bb.put((byte) 0x11);
bb.put((byte) 0x12);
bb.put((byte) 0x13);
bb.put((byte) 0x14);
bb.position(1).limit(5);
bb.mark();
System.out.println("Expect an exception here");
System.out.println("" + bb.order().toString() + ": " + bb.getLong());
}
}
/**/</source>



<pre class=codeResult>Expect an exception here
Exception in thread "main" java.nio.BufferUnderflowException
at java.nio.Buffer.nextGetIndex(Buffer.java:404)
at java.nio.HeapByteBuffer.getLong(HeapByteBuffer.java:387)
at MainClass.main(MainClass.java:25)</pre>

== This convenience method sets the position to 0 ==



<source lang="java">
import java.nio.ByteBuffer;
public class Main {
public static void main(String[] argv) throws Exception {
ByteBuffer buf = ByteBuffer.allocateDirect(10);
buf.rewind(); // remaining=7
}
}</source>



== Use FileChannel and ByteBuffer to Copy File ==



<source lang="java">
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
public class Main {
public static void main(String[] args) throws Exception {
String source = "s.txt";
String destination = "d.txt";
FileInputStream fis = new FileInputStream(source);
FileOutputStream fos = new FileOutputStream(destination);
FileChannel fci = fis.getChannel();
FileChannel fco = fos.getChannel();
ByteBuffer buffer = ByteBuffer.allocate(1024);
while (true) {
int read = fci.read(buffer);
if (read == -1)
break;
buffer.flip();
fco.write(buffer);
buffer.clear();
}
}
}</source>



== Use the absolute get(). ==



<source lang="java">
import java.nio.ByteBuffer;
public class Main {
public static void main(String[] argv) throws Exception {
ByteBuffer buf = ByteBuffer.allocateDirect(10);
byte b = buf.get(5); // position=0
}
}</source>



== Use the relative get() ==



<source lang="java">
import java.nio.ByteBuffer;
public class Main {
public static void main(String[] argv) throws Exception {
ByteBuffer buf = ByteBuffer.allocateDirect(10);
byte b = buf.get();
}
}</source>



== Use while loop to read a ByteBuffer ==



<source lang="java">
import java.nio.ByteBuffer;
public class MainClass {
public static void main(String[] args) {
ByteBuffer bb = ByteBuffer.wrap(new byte[] { 0, 0, 0, 0, 0, 0, 0, "a" });
bb.rewind();
System.out.println("Byte Buffer");
while (bb.hasRemaining())
System.out.println(bb.position() + " -> " + bb.get());
}
}
/**/</source>



<pre class=codeResult>Byte Buffer
0 -> 0
1 -> 0
2 -> 0
3 -> 0
4 -> 0
5 -> 0
6 -> 0
7 -> 97</pre>

← Предыдущая	Версия 05:20, 1 июня 2010
(нет различий)

Java Tutorial/File/ByteBuffer - История изменений

Admin: 1 версия

в 17:44, 31 мая 2010