multithreading - Unexpected output in concurrent file writing in java web -

i wrote simple java web application ,just included basic function register , sign in , changing password , others.

i don't use database. create file in app record users' information , database stuff.

i used jmeter stressing web application, register interface. jmeter shows result of 1000 thread right when information.txt , stores users' information, it's wrong because stores 700+ record :

but should include 1000 record, must somewhere wrong

i use singleton class write/read stuff, , add synchronized word class, insert() function used register record register information shown below: (a part of it)

public class database {  private static database database = null; private static file file = null;   public synchronized static database getinstance() {      if (database == null) {         database = new database();     }      return database; }   private database() {      string path = this.getclass().getclassloader().getresource("/")             .getpath() + "information.txt";     file = new file(path);      if (!file.exists()) {         try {             file.createnewfile();         } catch (ioexception ex) {             ex.printstacktrace();         }     }  } public void insert(string account, string password, string username) {      randomaccessfile infofile = null;      try {         infofile = new randomaccessfile(file, "rw");         string record;         long offset = 0;          while ((record = infofile.readline()) != null ) {             offset += record.getbytes().length+2;         }          infofile.seek(offset);         record = account+"|"+password+"|"+username+"\r\n";         infofile.write(record.getbytes());         infofile.close();      } catch (ioexception e) {         e.printstacktrace();      } {         if (infofile != null) {             try {                 infofile.close();             } catch (ioexception ex) {                 ex.printstacktrace();             }         }     }   } } 

the question why happened , synchronized thread safe, why lost many data , blank line inserted it, correct !

you synchronizing getinstance() method, not insert() method. makes retrieval of instance of database thread-safe, not write operation.