FileChannel's awesome way to read files

The power of FileChannel is that it maintains the current position of a file

 

See the dubbo source code, which demonstrates a way to read files through Channel

 

/*
 * Copyright 1999-2011 Alibaba Group.
 *  
 * Licensed 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 com.alibaba.dubbo.remoting.telnet.support.command;


import java.io.File;
import java.io.FileInputStream;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.text.SimpleDateFormat;
import java.util.Date;

import com.alibaba.dubbo.common.extension.Activate;
import com.alibaba.dubbo.common.logger.Level;
import com.alibaba.dubbo.common.logger.LoggerFactory;
import com.alibaba.dubbo.common.utils.StringUtils;
import com.alibaba.dubbo.remoting.Channel;
import com.alibaba.dubbo.remoting.telnet.TelnetHandler;
import com.alibaba.dubbo.remoting.telnet.support.Help;

/**
 * LogTelnetHandler
 * @author chao.liuc
 *
 */
@Activate
@Help(parameter = "level", summary = "Change log level or show log ", detail = "Change log level or show log")
public class LogTelnetHandler implements TelnetHandler {
    
    public static final String SERVICE_KEY = "telnet.log";

    public String telnet(Channel channel, String message) {
        long size = 0 ;
        File file = LoggerFactory.getFile();
        StringBuffer buf = new StringBuffer();
        if (message == null || message.trim().length() == 0) {
            buf.append("EXAMPLE: log error / log 100");
        }else {
            String str[] = message.split(" ");
            if (! StringUtils.isInteger(str[0])){
                LoggerFactory.setLevel(Level.valueOf(message.toUpperCase()));
            } else {
                int SHOW_LOG_LENGTH = Integer.parseInt(str[0]);
                
                if (file != null && file.exists()) {
                    try{
                        FileInputStream fis = new FileInputStream(file);
                        try {
	                        FileChannel filechannel = fis.getChannel();
	                        try {
		                        size = filechannel.size();
		                        ByteBuffer bb;
		                        if (size <= SHOW_LOG_LENGTH) {
		                            bb = ByteBuffer.allocate((int) size);
		                            filechannel.read(bb, 0);
		                        } else {
		                            int pos = (int) (size - SHOW_LOG_LENGTH);
		                            bb = ByteBuffer.allocate(SHOW_LOG_LENGTH);
		                            filechannel.read(bb, pos);
		                        }
		                        bb.flip();
		                        String content = new String(bb.array()).replace("<", "<")
		                        .replace(">", ">").replace("\n", "<br/><br/>");
		                        buf.append("\r\ncontent:"+content);
		                        
		                        buf.append("\r\nmodified:"+(new SimpleDateFormat("yyyy-MM-dd HH:mm:ss")
		                        .format(new Date(file.lastModified()))));
		                        buf.append("\r\nsize:"+size +"\r\n");
	                        } finally {
	                        	filechannel.close();
	                        }
                        } finally {
                        	fis.close();
                        }
                    }catch (Exception e) {
                        buf.append(e.getMessage());
                    }
                }else {
                    size = 0;
                    buf.append("\r\nMESSAGE: log file not exists or log appender is console .");
                }
            }
        }
        buf.append("\r\nCURRENT LOG LEVEL:"+ LoggerFactory.getLevel())
        .append("\r\nCURRENT LOG APPENDER:"+ (file == null ? "console" : file.getAbsolutePath()));
        return buf.toString();
    }

}