//类似于此
    public static void main(String[] args) throws Exception {
        BufferedReader reader = new BufferedReader(new FileReader("D:\\workspace\\test\\src\\test.txt"));
        boolean start = false;
        StringBuilder sb = new StringBuilder();
        while(true){
            String line = reader.readLine();
            if(line == null) break;
            if(!start){
                if(line.indexOf("AAA") != -1){
                    line = line.substring(line.indexOf("AAA"));
                    sb.append(line).append("\n");
                    start = true;
                    continue;
                }
            }else{
                if(line.indexOf("BBB") != -1){
                    line = line.substring(0, line.indexOf("BBB") + "BBB".length());
                    sb.append(line).append("\n");
                    start = false;
                    break;
                }else{
                    sb.append(line).append("\n");
                }
            }
        }
        System.out.println(sb.toString());
    }

------解决方案--------------------
import java.io.*;
public class Demo1 {
	public static String read()
	{
		FileReader fr=null;
		BufferedReader br=null;
		try
		{
			fr=new FileReader("d://Demo.txt");
			br=new BufferedReader(fr);
			String s="";
			StringBuffer str =new StringBuffer();
			while((s=br.readLine())!=null)
					{
				if(s.equals("AAA"))
				{
				while(!s.equals("BBB"))
				{
				
				str.append(s+"\r\n");
				s=br.readLine();
				}
				return str.append("BBB").toString();
				}
					}
			
		}
		catch(Exception e)
		{
			e.getStackTrace();
		}
		finally
		{
			try
			{
				br.close();
				fr.close();
			}
			catch(Exception e)
			{
				e.getStackTrace();
			}
		}
		return null;
	}
	public static void main(String[] args) {
		String s=read();
		System.out.println(s);

	}

}