[hadoop2.7.1]I/O之一步一步解析Text（基础知识及与String比较）
hadoop中的Text类，跟java中的String类很相似，在其定义的方法上，也多有相近之处，当然，由于用途的不同，两者之间还是有很大的区别的，那么，在分析Text类之前，先来回顾下java.lang.String类。
1、java中的String类：
字符串是常量；它们的值在创建之后不能更改。字符串缓冲区支持可变的字符串。因为String对象是不可变的，所以可以共享。例如：
Stringstr="abc";
等效于：
chardata[]={'a','b','c'};
Stringstr=newString(data);
下面给出了一些如何使用字符串的更多示例：
System.out.println("abc");
Stringcde="cde";
System.out.println("abc"+cde);
Stringc="abc".substring(2,3);
Stringd=cde.substring(1,2);
String类包括的方法可用于检查序列的单个字符、比较字符串、搜索字符串、提取子字符串、创建字符串副本并将所有字符全部转换为大写或小写。大小写映射基于Character类指定的Unicode标准版。
Java语言提供对字符串串联符号（"+"）以及将其他对象转换为字符串的特殊支持。字符串串联是通过StringBuilder（或StringBuffer）类及其append方法实现的。字符串转换是通过toString方法实现的，该方法由Object类定义，并可被Java中的所有类继承。有关字符串串联和转换的更多信息，请参阅Gosling、Joy和Steele合著的TheJavaLanguageSpecification。
除非另行说明，否则将null参数传递给此类中的构造方法或方法将抛出NullPointerException。
String表示一个UTF-16格式的字符串，其中的增补字符由代理项对表示（有关详细信息，请参阅Character类中的Unicode字符表示形式）。索引值是指char代码单元，因此增补字符在String中占用两个位置。
String类提供处理Unicode代码点（即字符）和Unicode代码单元（即char值）的方法。
2、Unicode
hadoop中的Text类和java中的String类都是使用标准的Unicode，但是在编码方式上却有不同之处，hadoop中的Text类使用修订的UTF-8，而java中的String类使用的是UTF-16。接下来，对于Unicode做一个较为详细的阐述。Unicode
Unicode（统一码、万国码、单一码）是一种在计算机上使用的字符编码。Unicode 是为了解决传统的字符编码方案的局限而产生的，它为每种语言中的每个字符设定了统一并且唯一的二进制编码，以满足跨语言、跨平台进行文本转换、处理的要求。Unicode涉及到两个步骤，首先是定义一个规范，给所有的字符指定一个唯一对应的数字，第二步才是怎么把字符对应的数字保存在计算机中，这才涉及到实际在计算机中占多少字节空间。UTF-8 、UTF-16、UTF-32
那么现在，我们的问题来了，UTF-8 、UTF-16、UTF-32之间到底有何区别呢？
于是又有个UTF-8，这里的8非常容易误导人，8不是指一个字节，难道一个字节表示一个字符？实际上不是。当用UTF-8时表示一个字符是可变的，有可能是用一个字节表示一个字符，也可能是两个，三个，四个，当然最多不能超过4个字节，具体根据字符对应的数字大小来确定。实际表示ASCII字符的UNICODE字符，将会编码成1个字节，并且UTF-8表示与ASCII字符表示是一样的。所有其他的UNICODE字符转化成UTF-8将需要至少2个字节。每个字节由一个换码序列开始。第一个字节由唯一的换码序列，由n位连续的1加一位0组成，首字节连续的1的个数表示字符编码所需的字节数。
现在UTF-8和UTF-16的优劣很容易就看出来了：如果全部英文或英文与其他文字混合，但英文占绝大部分，用UTF-8就比UTF-16节省了很多空间。
BYTEdata_utf8[]={0xE6,0xB1,0x89,0xE5,0xAD,0x97};//UTF-8编码
WORDdata_utf16[]={0x6c49,0x5b57};//UTF-16编码
DWORDdata_utf32[]={0x6c49,0x5b57};//UTF-32编码
这里用BYTE、WORD、DWORD分别表示无符号8位整数，无符号16位整数和无符号32位整数。UTF-8、UTF-16、UTF-32分别以BYTE、WORD、DWORD作为编码单位。“汉字”的UTF-8编码需要6个字节。“汉字”的UTF-16编码需要两个WORD，大小是4个字节。“汉字”的UTF-32编码需要两个DWORD，大小是8个字节。根据字节序的不同，UTF-16可以被实现为UTF-16LE或UTF-16BE，UTF-32可以被实现为UTF-32LE或UTF-32BE。
下面分别介绍UTF-8、UTF-16、UTF-32。
UTF-8（注意：Text使用修订的标准UTF-8编码）
UTF-8以字节为单位对Unicode进行编码。从Unicode到UTF-8的编码方式如下：
Unicode编码(16进制) ║ UTF-8 字节流(二进制)
000000 - 00007F ║ 0xxxxxxx
000080 - 0007FF ║ 110xxxxx 10xxxxxx
000800 - 00FFFF ║ 1110xxxx 10xxxxxx 10xxxxxx
010000 - 10FFFF ║ 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
UTF-8的特点是对不同范围的字符使用不同长度的编码。对于0x00-0x7F之间的字符，UTF-8编码与ASCII编码完全相同。UTF-8编码的最大长度是4个字节。从上表可以看出，4字节模板有21个x，即可以容纳21位二进制数字。Unicode的最大码位0x10FFFF也只有21位。
例1：“汉”字的Unicode编码是0x6C49。0x6C49在0x0800-0xFFFF之间，使用用3字节模板了：1110xxxx 10xxxxxx 10xxxxxx。将0x6C49写成二进制是：0110 1100 0100 1001， 用这个比特流依次代替模板中的x，得到：11100110 10110001 10001001，即E6 B1 89。
例2：Unicode编码0x20C30在0x010000-0x10FFFF之间，使用用4字节模板了：11110xxx 10xxxxxx 10xxxxxx 10xxxxxx。将0x20C30写成21位二进制数字（不足21位就在前面补0）：0 0010 0000 1100 0011 0000，用这个比特流依次代替模板中的x，得到：11110000 10100000 10110000 10110000，即F0 A0 B0 B0。
UTF-16
UTF-16编码以16位无符号整数为单位。我们把Unicode编码记作U。编码规则如下：
如果U<0x10000，U的UTF-16编码就是U对应的16位无符号整数（为书写简便，下文将16位无符号整数记作WORD）。
如果U≥0x10000，我们先计算U'=U-0x10000，然后将U'写成二进制形式：yyyyyyyyyyxxxxxxxxxx，U的UTF-16编码（二进制）就是：110110yyyyyyyyyy110111xxxxxxxxxx。
为什么U'可以被写成20个二进制位？Unicode的最大码位是0x10ffff，减去0x10000后，U'的最大值是0xfffff，所以肯定可以用20个二进制位表示。例如：Unicode编码0x20C30，减去0x10000后，得到0x10C30，写成二进制是：00010000110000110000。用前10位依次替代模板中的y，用后10位依次替代模板中的x，就得到：11011000010000111101110000110000，即0xD8430xDC30。
按照上述规则，Unicode编码0x10000-0x10FFFF的UTF-16编码有两个WORD，第一个WORD的高6位是110110，第二个WORD的高6位是110111。可见，第一个WORD的取值范围（二进制）是1101100000000000到1101101111111111，即0xD800-0xDBFF。第二个WORD的取值范围（二进制）是1101110000000000到1101111111111111，即0xDC00-0xDFFF。
为了将一个WORD的UTF-16编码与两个WORD的UTF-16编码区分开来，Unicode编码的设计者将0xD800-0xDFFF保留下来，并称为代理区（Surrogate）：
D800－DB7F ║ HighSurrogates ║ 高位替代
DB80－DBFF ║ HighPrivateUseSurrogates ║ 高位专用替代
DC00－DFFF ║ LowSurrogates ║ 低位替代
高位替代就是指这个范围的码位是两个WORD的UTF-16编码的第一个WORD。低位替代就是指这个范围的码位是两个WORD的UTF-16编码的第二个WORD。那么，高位专用替代是什么意思？我们来解答这个问题，顺便看看怎么由UTF-16编码推导Unicode编码。
如果一个字符的UTF-16编码的第一个WORD在0xDB80到0xDBFF之间，那么它的Unicode编码在什么范围内？我们知道第二个WORD的取值范围是0xDC00-0xDFFF，所以这个字符的UTF-16编码范围应该是0xDB800xDC00到0xDBFF0xDFFF。我们将这个范围写成二进制：
11011011100000001101110000000000-11011011111111111101111111111111
按照编码的相反步骤，取出高低WORD的后10位，并拼在一起，得到
11100000000000000000-11111111111111111111
即0xe0000-0xfffff，按照编码的相反步骤再加上0x10000，得到0xf0000-0x10ffff。这就是UTF-16编码的第一个WORD在0xdb80到0xdbff之间的Unicode编码范围，即平面15和平面16。因为Unicode标准将平面15和平面16都作为专用区，所以0xDB80到0xDBFF之间的保留码位被称作高位专用替代。
UTF-32
UTF-32编码以32位无符号整数为单位。Unicode的UTF-32编码就是其对应的32位无符号整数。
字节序
根据字节序的不同，UTF-16可以被实现为UTF-16LE或UTF-16BE，UTF-32可以被实现为UTF-32LE或UTF-32BE。例如：
Unicode编码 ║ UTF-16LE ║ UTF-16BE ║ UTF32-LE ║ UTF32-BE
0x006C49 ║ 496C ║ 6C49 ║ 496C0000 ║ 00006C49
0x020C30 ║ 43D830DC ║ D843DC30 ║ 300C0200 ║ 00020C30
Text类
hadoop2.7.1中的Text源码：
/**
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you under the Apache License, Version 2.0 (the
* "License"); you may not use this file except in compliance
* with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.hadoop.io;
import java.io.IOException;
import java.io.DataInput;
import java.io.DataOutput;
import java.nio.ByteBuffer;
import java.nio.CharBuffer;
import java.nio.charset.CharacterCodingException;
import java.nio.charset.Charset;
import java.nio.charset.CharsetDecoder;
import java.nio.charset.CharsetEncoder;
import java.nio.charset.CodingErrorAction;
import java.nio.charset.MalformedInputException;
import java.text.CharacterIterator;
import java.text.StringCharacterIterator;
import java.util.Arrays;
import org.apache.avro.reflect.Stringable;
import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;
/** This class stores text using standard UTF8 encoding. It provides methods
* to serialize, deserialize, and compare texts at byte level. The type of
* length is integer and is serialized using zero-compressed format. In
* addition, it provides methods for string traversal without converting the
* byte array to a string.
Also includes utilities for
* serializing/deserialing a string, coding/decoding a string, checking if a
* byte array contains valid UTF8 code, calculating the length of an encoded
* string.
*/
@Stringable
@InterfaceAudience.Public
@InterfaceStability.Stable
public class Text extends BinaryComparable
implements WritableComparable {
private static ThreadLocal ENCODER_FACTORY =
new ThreadLocal() {
@Override
protected CharsetEncoder initialValue() {
return Charset.forName("UTF-8").newEncoder().
onMalformedInput(CodingErrorAction.REPORT).
onUnmappableCharacter(CodingErrorAction.REPORT);
}
};
private static ThreadLocal DECODER_FACTORY =
new ThreadLocal() {
@Override
protected CharsetDecoder initialValue() {
return Charset.forName("UTF-8").newDecoder().
onMalformedInput(CodingErrorAction.REPORT).
onUnmappableCharacter(CodingErrorAction.REPORT);
}
};
private static final byte [] EMPTY_BYTES = new byte[0];
private byte[] bytes;
private int length;
public Text() {
bytes = EMPTY_BYTES;
}
/** Construct from a string.
*/
public Text(String string) {
set(string);
}
/** Construct from another text. */
public Text(Text utf8) {
set(utf8);
}
/** Construct from a byte array.
*/
public Text(byte[] utf8) {
set(utf8);
}
/**
* Get a copy of the bytes that is exactly the length of the data.
* See {@link #getBytes()} for faster access to the underlying array.
*/
public byte[] copyBytes() {
byte[] result = new byte[length];
System.arraycopy(bytes, 0, result, 0, length);
return result;
}
/**
* Returns the raw bytes; however, only data up to {@link #getLength()} is
* valid. Please use {@link #copyBytes()} if you
* need the returned array to be precisely the length of the data.
*/
@Override
public byte[] getBytes() {
return bytes;
}
/** Returns the number of bytes in the byte array */
@Override
public int getLength() {
return length;
}
/**
* Returns the Unicode Scalar Value (32-bit integer value)
* for the character at position. Note that this
* method avoids using the converter or doing String instantiation
* @return the Unicode scalar value at position or -1
* if the position is invalid or points to a
* trailing byte
*/
public int charAt(int position) {
if (position > this.length) return -1; // too long
if (position < 0) return -1; // duh.
ByteBuffer bb = (ByteBuffer)ByteBuffer.wrap(bytes).position(position);
return bytesToCodePoint(bb.slice());
}
public int find(String what) {
return find(what, 0);
}
/**
* Finds any occurence of what in the backing
* buffer, starting as position start. The starting
* position is measured in bytes and the return value is in
* terms of byte position in the buffer. The backing buffer is
* not converted to a string for this operation.
* @return byte position of the first occurence of the search
* string in the UTF-8 buffer or -1 if not found
*/
public int find(String what, int start) {
try {
ByteBuffer src = ByteBuffer.wrap(this.bytes,0,this.length);
ByteBuffer tgt = encode(what);
byte b = tgt.get();
src.position(start);
while (src.hasRemaining()) {
if (b == src.get()) { // matching first byte
src.mark(); // save position in loop
tgt.mark(); // save position in target
boolean found = true;
int pos = src.position()-1;
while (tgt.hasRemaining()) {
if (!src.hasRemaining()) { // src expired first
tgt.reset();
src.reset();
found = false;
break;
}
if (!(tgt.get() == src.get())) {
tgt.reset();
src.reset();
found = false;
break; // no match
}
}
if (found) return pos;
}
}
return -1; // not found
} catch (CharacterCodingException e) {
// can't get here
e.printStackTrace();
return -1;
}
}
/** Set to contain the contents of a string.
*/
public void set(String string) {
try {
ByteBuffer bb = encode(string, true);
bytes = bb.array();
length = bb.limit();
}catch(CharacterCodingException e) {
throw new RuntimeException("Should not have happened ", e);
}
}
/** Set to a utf8 byte array
*/
public void set(byte[] utf8) {
set(utf8, 0, utf8.length);
}
/** copy a text. */
public void set(Text other) {
set(other.getBytes(), 0, other.getLength());
}
/**
* Set the Text to range of bytes
* @param utf8 the data to copy from
* @param start the first position of the new string
* @param len the number of bytes of the new string
*/
public void set(byte[] utf8, int start, int len) {
setCapacity(len, false);
System.arraycopy(utf8, start, bytes, 0, len);
this.length = len;
}
/**
* Append a range of bytes to the end of the given text
* @param utf8 the data to copy from
* @param start the first position to append from utf8
* @param len the number of bytes to append
*/
public void append(byte[] utf8, int start, int len) {
setCapacity(length + len, true);
System.arraycopy(utf8, start, bytes, length, len);
length += len;
}
/**
* Clear the string to empty.
*
* Note: For performance reasons, this call does not clear the
* underlying byte array that is retrievable via {@link #getBytes()}.
* In order to free the byte-array memory, call {@link #set(byte[])}
* with an empty byte array (For example, new byte[0]).
*/
public void clear() {
length = 0;
}
/*
* Sets the capacity of this Text object to at least
* len bytes. If the current buffer is longer,
* then the capacity and existing content of the buffer are
* unchanged. If len is larger
* than the current capacity, the Text object's capacity is
* increased to match.
* @param len the number of bytes we need
* @param keepData should the old data be kept
*/
private void setCapacity(int len, boolean keepData) {
if (bytes == null || bytes.length < len) {
if (bytes != null && keepData) {
bytes = Arrays.copyOf(bytes, Math.max(len,length << 1));
} else {
bytes = new byte[len];
}
}
}
/**
* Convert text back to string
* @see java.lang.Object#toString()
*/
@Override
public String toString() {
try {
return decode(bytes, 0, length);
} catch (CharacterCodingException e) {
throw new RuntimeException("Should not have happened " , e);
}
}
/** deserialize
*/
@Override
public void readFields(DataInput in) throws IOException {
int newLength = WritableUtils.readVInt(in);
readWithKnownLength(in, newLength);
}
public void readFields(DataInput in, int maxLength) throws IOException {
int newLength = WritableUtils.readVInt(in);
if (newLength < 0) {
throw new IOException("tried to deserialize " + newLength +
" bytes of data! newLength must be non-negative.");
} else if (newLength >= maxLength) {
throw new IOException("tried to deserialize " + newLength +
" bytes of data, but maxLength = " + maxLength);
}
readWithKnownLength(in, newLength);
}
/** Skips over one Text in the input. */
public static void skip(DataInput in) throws IOException {
int length = WritableUtils.readVInt(in);
WritableUtils.skipFully(in, length);
}
/**
* Read a Text object whose length is already known.
* This allows creating Text from a stream which uses a different serialization
* format.
*/
public void readWithKnownLength(DataInput in, int len) throws IOException {
setCapacity(len, false);
in.readFully(bytes, 0, len);
length = len;
}
/** serialize
* write this object to out
* length uses zero-compressed encoding
* @see Writable#write(DataOutput)
*/
@Override
public void write(DataOutput out) throws IOException {
WritableUtils.writeVInt(out, length);
out.write(bytes, 0, length);
}
public void write(DataOutput out, int maxLength) throws IOException {
if (length > maxLength) {
throw new IOException("data was too long to write! Expected " +
"less than or equal to " + maxLength + " bytes, but got " +
length + " bytes.");
}
WritableUtils.writeVInt(out, length);
out.write(bytes, 0, length);
}
/** Returns true iff o is a Text with the same contents. */
@Override
public boolean equals(Object o) {
if (o instanceof Text)
return super.equals(o);
return false;
}
@Override
public int hashCode() {
return super.hashCode();
}
/** A WritableComparator optimized for Text keys. */
public static class Comparator extends WritableComparator {
public Comparator() {
super(Text.class);
}
@Override
public int compare(byte[] b1, int s1, int l1,
byte[] b2, int s2, int l2) {
int n1 = WritableUtils.decodeVIntSize(b1[s1]);
int n2 = WritableUtils.decodeVIntSize(b2[s2]);
return compareBytes(b1, s1+n1, l1-n1, b2, s2+n2, l2-n2);
}
}
static {
// register this comparator
WritableComparator.define(Text.class, new Comparator());
}
/// STATIC UTILITIES FROM HERE DOWN
/**
* Converts the provided byte array to a String using the
* UTF-8 encoding. If the input is malformed,
* replace by a default value.
*/
public static String decode(byte[] utf8) throws CharacterCodingException {
return decode(ByteBuffer.wrap(utf8), true);
}
public static String decode(byte[] utf8, int start, int length)
throws CharacterCodingException {
return decode(ByteBuffer.wrap(utf8, start, length), true);
}
/**
* Converts the provided byte array to a String using the
* UTF-8 encoding. If replace is true, then
* malformed input is replaced with the
* substitution character, which is U+FFFD. Otherwise the
* method throws a MalformedInputException.
*/
public static String decode(byte[] utf8, int start, int length, boolean replace)
throws CharacterCodingException {
return decode(ByteBuffer.wrap(utf8, start, length), replace);
}
private static String decode(ByteBuffer utf8, boolean replace)
throws CharacterCodingException {
CharsetDecoder decoder = DECODER_FACTORY.get();
if (replace) {
decoder.onMalformedInput(
java.nio.charset.CodingErrorAction.REPLACE);
decoder.onUnmappableCharacter(CodingErrorAction.REPLACE);
}
String str = decoder.decode(utf8).toString();
// set decoder back to its default value: REPORT
if (replace) {
decoder.onMalformedInput(CodingErrorAction.REPORT);
decoder.onUnmappableCharacter(CodingErrorAction.REPORT);
}
return str;
}
/**
* Converts the provided String to bytes using the
* UTF-8 encoding. If the input is malformed,
* invalid chars are replaced by a default value.
* @return ByteBuffer: bytes stores at ByteBuffer.array()
* and length is ByteBuffer.limit()
*/
public static ByteBuffer encode(String string)
throws CharacterCodingException {
return encode(string, true);
}
/**
* Converts the provided String to bytes using the
* UTF-8 encoding. If replace is true, then
* malformed input is replaced with the
* substitution character, which is U+FFFD. Otherwise the
* method throws a MalformedInputException.
* @return ByteBuffer: bytes stores at ByteBuffer.array()
* and length is ByteBuffer.limit()
*/
public static ByteBuffer encode(String string, boolean replace)
throws CharacterCodingException {
CharsetEncoder encoder = ENCODER_FACTORY.get();
if (replace) {
encoder.onMalformedInput(CodingErrorAction.REPLACE);
encoder.onUnmappableCharacter(CodingErrorAction.REPLACE);
}
ByteBuffer bytes =
encoder.encode(CharBuffer.wrap(string.toCharArray()));
if (replace) {
encoder.onMalformedInput(CodingErrorAction.REPORT);
encoder.onUnmappableCharacter(CodingErrorAction.REPORT);
}
return bytes;
}
static final public int DEFAULT_MAX_LEN = 1024 * 1024;
/** Read a UTF8 encoded string from in
*/
public static String readString(DataInput in) throws IOException {
return readString(in, Integer.MAX_VALUE);
}
/** Read a UTF8 encoded string with a maximum size
*/
public static String readString(DataInput in, int maxLength)
throws IOException {
int length = WritableUtils.readVIntInRange(in, 0, maxLength);
byte [] bytes = new byte[length];
in.readFully(bytes, 0, length);
return decode(bytes);
}
/** Write a UTF8 encoded string to out
*/
public static int writeString(DataOutput out, String s) throws IOException {
ByteBuffer bytes = encode(s);
int length = bytes.limit();
WritableUtils.writeVInt(out, length);
out.write(bytes.array(), 0, length);
return length;
}
/** Write a UTF8 encoded string with a maximum size to out
*/
public static int writeString(DataOutput out, String s, int maxLength)
throws IOException {
ByteBuffer bytes = encode(s);
int length = bytes.limit();
if (length > maxLength) {
throw new IOException("string was too long to write! Expected " +
"less than or equal to " + maxLength + " bytes, but got " +
length + " bytes.");
}
WritableUtils.writeVInt(out, length);
out.write(bytes.array(), 0, length);
return length;
}
// states for validateUTF8
private static final int LEAD_BYTE = 0;
private static final int TRAIL_BYTE_1 = 1;
private static final int TRAIL_BYTE = 2;
/**
* Check if a byte array contains valid utf-8
* @param utf8 byte array
* @throws MalformedInputException if the byte array contains invalid utf-8
*/
public static void validateUTF8(byte[] utf8) throws MalformedInputException {
validateUTF8(utf8, 0, utf8.length);
}
/**
* Check to see if a byte array is valid utf-8
* @param utf8 the array of bytes
* @param start the offset of the first byte in the array
* @param len the length of the byte sequence
* @throws MalformedInputException if the byte array contains invalid bytes
*/
public static void validateUTF8(byte[] utf8, int start, int len)
throws MalformedInputException {
int count = start;
int leadByte = 0;
int length = 0;
int state = LEAD_BYTE;
while (count < start+len) {
int aByte = utf8[count] & 0xFF;
switch (state) {
case LEAD_BYTE:
leadByte = aByte;
length = bytesFromUTF8[aByte];
switch (length) {
case 0: // check for ASCII
if (leadByte > 0x7F)
throw new MalformedInputException(count);
break;
case 1:
if (leadByte < 0xC2 || leadByte > 0xDF)
throw new MalformedInputException(count);
state = TRAIL_BYTE_1;
break;
case 2:
if (leadByte < 0xE0 || leadByte > 0xEF)
throw new MalformedInputException(count);
state = TRAIL_BYTE_1;
break;
case 3:
if (leadByte < 0xF0 || leadByte > 0xF4)
throw new MalformedInputException(count);
state = TRAIL_BYTE_1;
break;
default:
// too long! Longest valid UTF-8 is 4 bytes (lead + three)
// or if < 0 we got a trail byte in the lead byte position
throw new MalformedInputException(count);
} // switch (length)
break;
case TRAIL_BYTE_1:
if (leadByte == 0xF0 && aByte < 0x90)
throw new MalformedInputException(count);
if (leadByte == 0xF4 && aByte > 0x8F)
throw new MalformedInputException(count);
if (leadByte == 0xE0 && aByte < 0xA0)
throw new MalformedInputException(count);
if (leadByte == 0xED && aByte > 0x9F)
throw new MalformedInputException(count);
// falls through to regular trail-byte test!!
case TRAIL_BYTE:
if (aByte < 0x80 || aByte > 0xBF)
throw new MalformedInputException(count);
if (--length == 0) {
state = LEAD_BYTE;
} else {
state = TRAIL_BYTE;
}
break;
default:
break;
} // switch (state)
count++;
}
}
/**
* Magic numbers for UTF-8. These are the number of bytes
* that follow a given lead byte. Trailing bytes
* have the value -1. The values 4 and 5 are presented in
* this table, even though valid UTF-8 cannot include the
* five and six byte sequences.
*/
static final int[] bytesFromUTF8 =
{ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0,
// trail bytes
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 3,
3, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 5, 5, 5, 5 };
/**
* Returns the next code point at the current position in
* the buffer. The buffer's position will be incremented.
* Any mark set on this buffer will be changed by this method!
*/
public static int bytesToCodePoint(ByteBuffer bytes) {
bytes.mark();
byte b = bytes.get();
bytes.reset();
int extraBytesToRead = bytesFromUTF8[(b & 0xFF)];
if (extraBytesToRead < 0) return -1; // trailing byte!
int ch = 0;
switch (extraBytesToRead) {
case 5: ch += (bytes.get() & 0xFF); ch <<= 6; /* remember, illegal UTF-8 */
case 4: ch += (bytes.get() & 0xFF); ch <<= 6; /* remember, illegal UTF-8 */
case 3: ch += (bytes.get() & 0xFF); ch <<= 6;
case 2: ch += (bytes.get() & 0xFF); ch <<= 6;
case 1: ch += (bytes.get() & 0xFF); ch <<= 6;
case 0: ch += (bytes.get() & 0xFF);
}
ch -= offsetsFromUTF8[extraBytesToRead];
return ch;
}
static final int offsetsFromUTF8[] =
{ 0x00000000, 0x00003080,
0x000E2080, 0x03C82080, 0xFA082080, 0x82082080 };
/**
* For the given string, returns the number of UTF-8 bytes
* required to encode the string.
* @param string text to encode
* @return number of UTF-8 bytes required to encode
*/
public static int utf8Length(String string) {
CharacterIterator iter = new StringCharacterIterator(string);
char ch = iter.first();
int size = 0;
while (ch != CharacterIterator.DONE) {
if ((ch >= 0xD800) && (ch < 0xDC00)) {
// surrogate pair?
char trail = iter.next();
if ((trail > 0xDBFF) && (trail < 0xE000)) {
// valid pair
size += 4;
} else {
// invalid pair
size += 3;
iter.previous(); // rewind one
}
} else if (ch < 0x80) {
size++;
} else if (ch < 0x800) {
size += 2;
} else {
// ch < 0x10000, that is, the largest char value
size += 3;
}
ch = iter.next();
}
return size;
}
}
下面是Text类实现的方法：
| Modifier and Type | Method and Description |
|---|---|
void |
append(byte[]utf8, intstart, intlen)
Append a range of bytes to the end of the given text
|
static int |
bytesToCodePoint(ByteBufferbytes)
Returns the next code point at the current position in the buffer.
|
int |
charAt(intposition)
Returns the Unicode Scalar Value (32-bit integer value) for the character at position.
|
void |
clear()
Clear the string to empty.
|
byte[] |
copyBytes()
Get a copy of the bytes that is exactly the length of the data.
|
static String |
decode(byte[]utf8)
Converts the provided byte array to a String using the UTF-8 encoding.
|
static String |
decode(byte[]utf8, intstart, intlength) |
static String |
decode(byte[]utf8, intstart, intlength, booleanreplace)
Converts the provided byte array to a String using the UTF-8 encoding.
|
static ByteBuffer |
encode(Stringstring)
Converts the provided String to bytes using the UTF-8 encoding.
|
static ByteBuffer |
encode(Stringstring, booleanreplace)
Converts the provided String to bytes using the UTF-8 encoding.
|
boolean |
equals(Objecto)
Returns true iff o is a Text with the same contents.
|
int |
find(Stringwhat) |
int |
find(Stringwhat, intstart)
Finds any occurence of what in the backing buffer, starting as position start.
|
byte[] |
getBytes()
Returns the raw bytes; however, only data up to getLength() is valid.
|
int |
getLength()
Returns the number of bytes in the byte array
|
int |
hashCode()
Return a hash of the bytes returned from {#getBytes()}.
|
void |
readFields(DataInputin)
deserialize
|
void |
readFields(DataInputin, intmaxLength) |
static String |
readString(DataInputin)
Read a UTF8 encoded string from in
|
static String |
readString(DataInputin, intmaxLength)
Read a UTF8 encoded string with a maximum size
|
void |
readWithKnownLength(DataInputin, intlen)
Read a Text object whose length is already known.
|
void |
set(byte[]utf8)
Set to a utf8 byte array
|
void |
set(byte[]utf8, intstart, intlen)
Set the Text to range of bytes
|
void |
set(Stringstring)
Set to contain the contents of a string.
|
void |
set(Textother)
copy a text.
|
static void |
skip(DataInputin)
Skips over one Text in the input.
|
String |
toString()
Convert text back to string
|
static int |
utf8Length(Stringstring)
For the given string, returns the number of UTF-8 bytes required to encode the string.
|
static void |
validateUTF8(byte[]utf8)
Check if a byte array contains valid utf-8
|
static void |
validateUTF8(byte[]utf8, intstart, intlen)
Check to see if a byte array is valid utf-8
|
void |
write(DataOutputout)
serialize write this object to out length uses zero-compressed encoding
|
void |
write(DataOutputout, intmaxLength) |
static int |
writeString(DataOutputout,Strings)
Write a UTF8 encoded string to out
|
static int |
writeString(DataOutputout,Strings, intmaxLength)
Write a UTF8 encoded string with a maximum size to out
|
为对上面阐述进行验证，接下来将列举一个测试实例，并进行分析。
参考：
http://hadoop.apache.org/docs/current/