由於servlet本身就是併發處理的，因此我們只需要處理好對應的呼叫函式就可以了。

考慮到對效率有要求，因此使用一個StringBuffer做緩衝區，將資料儲存在記憶體中，然後定時將資料寫入檔案。基本思路是先將檔名儲存在記憶體中，將url的filename與記憶體中的filename相比較，如果匹配，則向StringBuffer中寫入content內容，如果不匹配，則直接建立檔案，寫入content內容。使用StringBuffer的length作為判斷寫入檔案的條件。
以下是實現程式碼

servlet，簡單的實現了一個對url的控制，便於測試

package savelog.packagename.com;

import
 
 java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class SaveLog
 */
 

@WebServlet("/SaveLog")
public class SaveLog extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#HttpServlet()
     */
    public SaveLog() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
 

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        request.setCharacterEncoding("UTF-8");
        PrintWriter writer = response.getWriter();
        writer.println("DIR1:"+request.getParameter("dir1"));
        writer.println("DIR2:"+request.getParameter("dir2"));
        writer.println("DIR3:"+request.getParameter("dir3"));
        writer.println("filename:"+request.getParameter("filename"));
        writer.println("content:"+request.getParameter("content"));
        writer.println(request.getCharacterEncoding());

        if(request.getParameter("dir1")==null){
            writer.println("dir1 muxt exist");
            return;
        }

        //String path2=request.getParameter("dir1")+"\\"+request.getParameter("dir2")+"\\"+request.getParameter("dir3");
        StringBuilder path=new StringBuilder();
        path.append(request.getParameter("dir1"));
        if(request.getParameter("dir2")!=null&&!request.getParameter("dir2").equals(""))
            path.append("\\"+request.getParameter("dir2"));
        if(request.getParameter("dir3")!=null&&!request.getParameter("dir3").equals(""))
            path.append("\\"+request.getParameter("dir3"));
        String filename=request.getParameter("filename");
        String temp=request.getParameter("content");
        String content=new String(temp.getBytes("ISO-8859-1"),"UTF-8");
        writer.println("Path:"+path);
        writer.println("savePath:"+Data.saveLog(path.toString(), filename, content));

    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

Global類，定義全域性變數，儲存在記憶體中。

package savelog.packagename.com;

public class Global {

    /*寫日誌緩衝區*/
    public static StringBuffer stringBuffer=new StringBuffer();
    /*filename標誌*/
    public static String filename=new String("");
    /*根路徑*/
    public static final String rootPath = "e:\\logroot";

}

Date類，業務邏輯類，實現了儲存的業務邏輯

package savelog.packagename.com;

import java.io.BufferedWriter;
import java.io.File;
import java.io.FileOutputStream;
import java.io.OutputStreamWriter;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;


public class Data {
    /**
     * 訪問方法(同步)
     * @param path
     * @param filename
     * @param content
     * @return
     */
    public static synchronized String saveLog(String path, String filename, String content){
        if(Global.filename.equals("")){
            //第一次訪問，建立檔案
            Global.filename=filename;
            String savePath=createFile(path,filename,content);
            Global.stringBuffer.setLength(0);
            return savePath;
        }
        if(Global.filename.equals(filename)){
            //追加入已有檔案
            Global.stringBuffer.append(getTime()+"\r\n"+content+"\r\n");
            if (Global.stringBuffer.length()>10000) {
                writelog(path, Global.filename, Global.stringBuffer.toString());
                Global.stringBuffer.setLength(0);
            }
            return Global.stringBuffer.toString();
        }else {
            //更換檔名
            if(Global.stringBuffer.length()>0){
                writelog(path, Global.filename, Global.stringBuffer.toString());
                Global.stringBuffer.setLength(0);
            }
            String savePath=createFile(path,filename,content);
            Global.filename=filename;
            return savePath;
        }
    }

    /**
     * 建立新的檔案路徑，並寫入當前 content
     * @param path
     * @throws Throwable 
     */
    private static String createFile(String path,String filename,String content){
        String saveDirPath=Global.rootPath+"\\"+path+"\\"+getDate();
        File Dirs = new File(saveDirPath);
        Dirs.mkdirs();
        String saveFilePath=saveDirPath+"\\"+filename+".log";
        BufferedWriter writer = null; 
        try {
            writer = new BufferedWriter(new OutputStreamWriter(new FileOutputStream(saveFilePath, true),"UTF-8"));
            writer.write(getTime()+"\r\n"+content+"\r\n");   
            writer.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
        return saveFilePath;

    }

    /**
     * 將StringBuffer中的內容寫入到file
     * @param path
     * @param content
     */
    private static void writelog(String path,String filename,String content){
        BufferedWriter writer = null; 
        String saveFilePath=Global.rootPath+"\\"+path+"\\"+getDate()+"\\"+filename+".log";
//      處理執行中檔案被刪除的情況
//      if(!new File(saveFilePath).exists())
//          new File(Global.rootPath+"\\"+path+"\\"+getDate()).mkdirs();
        try {
            writer = new BufferedWriter(new OutputStreamWriter(new FileOutputStream(saveFilePath, true),"UTF-8"));
            writer.write(content);   
            writer.close();
        } catch (Exception e) {
            e.printStackTrace();
        }   
    }

    /**
     * 獲得當前日期
     * @return 例：2015-10-11
     */
    private static String getDate(){
        Calendar calendar = Calendar.getInstance();
        int year = calendar.get(Calendar.YEAR);
        int month = calendar.get(Calendar.MONTH);
        int day = calendar.get(Calendar.DAY_OF_MONTH);
        String dateStr = "" + year + "-" + month + "-" + day;
        return dateStr;
    }

    /**
     * 獲得當前時間 例：2015-10-11 15:00:01
     * @return
     */
    private static String getTime(){
        SimpleDateFormat simpleDateFormat=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        return simpleDateFormat.format(new Date().getTime());
    }

}

併發的處理，簡單的處理了併發，使用synchronized關鍵字處理了saveLog方法，確保每次只有一個執行緒可以訪問該方法，這樣就保證了該方法的安全性，避免在多執行緒的狀態下出現不可預期的結果。

效能測試：
使用apache ab進行壓力測試，10000次訪問的時間是3.2s左右，完全可以滿足效能要求。

不足：每次只能處理一個url，如果併發多個url，分佈在多個檔案中，效能太低，不能滿足需求。