nio~Charset

*介绍nio的字符处理功能,实现将buffer的内容装换为各种字符编码格式.

*示例,演示了如何使用Charset编码和解码

/**
 * Feb 25, 2011 by dzh
 */
package nio.charset;

import java.nio.ByteBuffer;
import java.nio.CharBuffer;
import java.nio.charset.Charset;
import java.nio.charset.CharsetDecoder;
import java.nio.charset.CharsetEncoder;
import java.nio.charset.CoderResult;
import java.util.Iterator;

/**
 * @author dzh
 * 
 */
public class CharsetTest {

	public static void main(String[] args) throws Exception {
		System.out.println("*******************************************");
		if(Charset.isSupported("gbk"))
			System.out.println("GBK is supported");
		
		System.out.println("The default charset of this java jvm is "+Charset.defaultCharset().displayName());
		System.out.println("*******************************************");
		Iterator<String> iter =Charset.availableCharsets().keySet().iterator();
		while(iter.hasNext())
			System.out.println(iter.next());
		System.out.println("*******************************************");
		
		Charset charset =Charset.forName("gbk");
		CharBuffer inBuf0 =CharBuffer.wrap("光福香雪海,踏春寻梅.");
		CharBuffer inBuf1 =CharBuffer.wrap("湖光山色,洞天福地");
		ByteBuffer outBuf =ByteBuffer.allocate(1000);
		
		//encoder inBuf0
		CharsetEncoder encoder =charset.newEncoder();
		CoderResult coderResult =encoder.encode(inBuf0, outBuf, false);//false设置encoder的状态为未完成
		if(isInvalid(coderResult)){ 
			System.out.println(coderResult.toString());
			return; 
		}
		//encode inBuf1
		coderResult =encoder.encode(inBuf1, outBuf, true); //true告知encoder,编码工作结束
		if(isInvalid(coderResult)){
			System.out.println(coderResult.toString());
			return;
		}
		encoder.flush(outBuf); //设置编码器的完成状态等
		outBuf.flip();
		
		//decoder
		CharsetDecoder decoder =charset.newDecoder();
		CharBuffer restoredBuf =decoder.decode(outBuf);
		System.out.println(restoredBuf.toString()); //输出解码后的字符串
	}
	
	/**
	 * 判断不合法状态
	 * @param coderResult
	 * @return
	 */
	public static boolean isInvalid(CoderResult coderResult){
		if(coderResult.isError()||coderResult==CoderResult.OVERFLOW)
			return true;
		return false;
	}

}

---CharsetEncoder和CharsetDecoder都有2种操作形式,

以encoder为例,[1]CharsetEncoder.encode(CharBuffer in, ByteBuffer out, boolean endOfInput)和[2]CharsetEncoder.encode(CharBuffer in)

从源码可见,[1]有可以复用out,[2]则每次产生新的out buffer.上面的示例使用的是[1]方式,多次编码时,更加有效率,但是注意要调用flush().

*CharsetEncoder

---编码器有4个内部状态,表示它的工作状态.

调用一些方法会改变这些状态,同时一些方法的执行是需要特定的状态的,如flush()需要state == ST_END.

---CharsetEncoder.reset(),当重新使用encoder时调用它,"clearing any charset-specific internal state",state = ST_RESET.

*CoderResult

---记录编码和解码的结果状态,有5个状态

private static final int CR_UNDERFLOW  = 0;
private static final int CR_OVERFLOW   = 1;
private static final int CR_ERROR_MIN  = 2;
private static final int CR_MALFORMED  = 2;
private static final int CR_UNMAPPABLE = 3;

---CoderResult.UNDERFLOW

either the input buffer has been completely consumed or, if the input buffer is not yet empty,that additional input is required.

---CoderResult.OVERFLOW 

there is insufficient room in the output buffer,这个情况也属于错误,但isError()不检测这样情况

---isError

public boolean isError() {
    return (type >= CR_ERROR_MIN);

}

*总结

---通过示例说明了nio如何处理字节编码的一般方式.

---内部状态的设计风格,值得学习.