代码如下

package com.wenhaofan.common.kit;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;

import com.jfinal.kit.PropKit;

/**
* @author 作者:范文皓
* @createDate 创建时间:2018年9月13日 下午3:59:07
*/
public class PropertyKit {
	public static  void main(String[] args) {
		String path=FileKit.class.getResource("/blog_config.txt").getPath() ;
		path=path.substring(1, path.length());
		replace(path,"theme","newTheme");
		String theme=PropKit.use("blog_config.txt").get("theme");
		System.out.println(theme);
	 
		
	}
	public static void replace(String path,String key,String newValue) {
	       String temp = "";
	        try {
	            File file = new File(path);
	            FileInputStream fis = new FileInputStream(file);
	            InputStreamReader isr = new InputStreamReader(fis);
	            BufferedReader br = new BufferedReader(isr);
	            StringBuffer buf = new StringBuffer();

	            // 保存该行前面的内容
	            while ( (temp = br.readLine()) != null) {
	            	
	            	boolean isMath=StrKit.filterNull(temp).split("=")[0].equals(key);
	            	
	                if(isMath){
	                    buf = buf.append(key+"="+newValue);
	                }else{
	                    buf = buf.append(temp);
	                }
	                buf = buf.append(System.getProperty("line.separator"));
	            }

	            br.close();
	            FileOutputStream fos = new FileOutputStream(file);
	            PrintWriter pw = new PrintWriter(fos);
	            pw.write(buf.toString().toCharArray());
	            pw.flush();
	            pw.close();
	        } catch (IOException e) {
	            e.printStackTrace();
	        }
	    }
}

实现思路

StringBuffer中,

    如果当前行号为指定行号则添加替换的内容,否则添加原内容

    然后将StringBuffer中的内容覆盖写入文件