InputStreamReader

InputStreamReader可以将字节流转为字符流，

 构造函数
 public InputStreamReader(InputStream in) {
 		//设置对象锁
        super(in);
        try {
        	//确定解码格式，分配缓冲区
            sd = StreamDecoder.forInputStreamReader(in, this, (String)null); 
        } catch (UnsupportedEncodingException e) {
            // The default encoding should always be available
            throw new Error(e);
        }
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  读取字符
  public int read() throws IOException {
     return read0();
  }  
  private int read0() throws IOException {
        synchronized (lock) {

            // Return the leftover char, if there is one
            if (haveLeftoverChar) {
                haveLeftoverChar = false;
                return leftoverChar;
            }
			//这里一次读取两个字符
            char cb[] = new char[2];
            int n = read(cb, 0, 2);
            switch (n) {
            case -1:
                return -1;
            case 2:
            //读取到两个字符，保存第二个，
                leftoverChar = cb[1];
                haveLeftoverChar = true;
            case 1:
            //返回第一个字符
                return cb[0];
            default:
                assert false : n;
                return -1;
            }
        }
  } 

  	
    public int read(char cbuf[], int offset, int length) throws IOException {
        int off = offset;
        int len = length;
        synchronized (lock) {
            ensureOpen();
            if ((off < 0) || (off > cbuf.length) || (len < 0) ||
                ((off + len) > cbuf.length) || ((off + len) < 0)) {
                throw new IndexOutOfBoundsException();
            }
            if (len == 0)
                return 0;
			
            int n = 0;
			//如果有之前的剩余字符未读取
            if (haveLeftoverChar) {
                // 拷贝到数组
                cbuf[off] = leftoverChar;
                off++; len--;
                haveLeftoverChar = false;
                n = 1;
                //读完了，返回
                if ((len == 0) || !implReady())
                    return n;
            }
			//还需要读取一个字符
            if (len == 1) {
                int c = read0();
                if (c == -1)
                    return (n == 0) ? -1 : n;
                cbuf[off] = (char)c;
                return n + 1;
            }
			//读取off+len个字符，以off为头，拷贝过去
            return n + implRead(cbuf, off, off + len);
        }
    }

	 int implRead(char[] cbuf, int off, int end) throws IOException {
		
        assert (end - off > 1);
		//包装cbuf字符数组
        CharBuffer cb = CharBuffer.wrap(cbuf, off, end - off);
        if (cb.position() != 0)
        // Ensure that cb[0] == cbuf[off]
        cb = cb.slice();

        boolean eof = false;
        for (;;) {
        //进行字节流到字符流的解码
        CoderResult cr = decoder.decode(bb, cb, eof);
        if (cr.isUnderflow()) {
            if (eof)
                break;
            if (!cb.hasRemaining())
                break;
            if ((cb.position() > 0) && !inReady())
                break;          // Block at most once
            int n = readBytes();
            if (n < 0) {
                eof = true;
                if ((cb.position() == 0) && (!bb.hasRemaining()))
                    break;
                decoder.reset();
            }
            continue;
        }
        if (cr.isOverflow()) {
            assert cb.position() > 0;
            break;
        }
        cr.throwException();
        }

        if (eof) {
        // ## Need to flush decoder
        decoder.reset();
        }

        if (cb.position() == 0) {
            if (eof)
                return -1;
            assert false;
        }
        return cb.position();
    }

最后

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