java - Synchronized writing and reading in file with not join object variable of file -

i'm trying implement threads concurrency in java. consists of trying write in file last time. there 2 threads: a - creating file , checking if right line in file , b - searching file , trying rewrite file “good” line. “winner” thread must have string in file. thread checks if file has line, file has 1 line. threads have file path.

public class implements runnable { private file file; private thread t;  public a(string patch,string filename) {     t = new thread(this);     createfile(patch, filename);     //t.setdaemon(true);     t.start(); }  @override public void run() {     bufferedreader reader;     while (!thread.currentthread().isinterrupted()) {         try {             reader = new bufferedreader(new inputstreamreader(new fileinputstream(file)));             if (reader.readline().charat(0) == 'b') {                 system.out.println("a try took file: " + file.getname());                 write();             } else {                 system.out.println("a took file: " + file.getname());             }         } catch (filenotfoundexception e)         {             system.out.println("file read a" + e.tostring());         }         catch (ioexception e)         {             system.out.println("file read a"+e.tostring());         }     } }   private void write() {     try {         printwriter printwriter = new printwriter(file);         printwriter.println("a took file: " + file.getname());         system.out.println("a took file: " + file.getname());         printwriter.close();     } catch (exception e) {         system.out.println("file write a");     } }  public file createfile(string patch,string filename) {     file file = new file(patch,filename+".txt");     try {         printwriter printwriter = new printwriter(file);         printwriter.println("a took file: " + file.getname());         system.out.println("a took file: " + file.getname());         printwriter.close();     } catch (exception e) {         system.out.println("file create a");     }     return file; } }  public class b implements runnable { private file file; private thread t;  public b(string patch,string filename) {     t = new thread(this);     //t.setdaemon(true);     findfile(patch, filename);     t.start(); }  @override public void run() {     bufferedreader reader;     while (!thread.currentthread().isinterrupted()) {         try {             reader = new bufferedreader(new inputstreamreader(new fileinputstream(file)));             if (reader.readline().charat(0) == 'a') {                 system.out.println("b try took file: " + file.getname());                 write();             } else {                 system.out.println("b took file: " + file.getname());             }         } catch (filenotfoundexception e)         {             system.out.println("file read b" + e.tostring());         }         catch (ioexception e)         {             system.out.println("file read b"+e.tostring());         }     } }   private void write() {     try {         printwriter printwriter = new printwriter(file);         printwriter.println("b took file: " + file.getname());         system.out.println("b took file: " + file.getname());         printwriter.close();     } catch (exception e) {         system.out.println("file write b");     } }  public file findfile(string patch,string filename) {     file file= null;     file folder = new file(patch);     file[] listoffiles = folder.listfiles();     bufferedreader reader;     (int = 0; < listoffiles.length; i++) {         file = listoffiles[i];         if (file.getname().equals(filename + ".txt")) {             break;         }     }     return file; } } 

i want synchronize in way access file in threads. in code have java.lang.nullpointerexception when use readline(), think because threads don’t have synchronized access file (after every finished operation file must have 1 line). can’t use synchronized method or block, because threads don’t have joint variable of file. there way of making synchronized writing , reading in file?

you can use reentrantreadwritelock synchronize. parse same lock synchronize both , b. here have modified b. can same well.

public static class b implements runnable {         private file file;         private thread t;         private reentrantreadwritelock lock;          public b(string patch,string filename, reentrantreadwritelock lock)         {             t = new thread(this);             this.lock = lock;             //t.setdaemon(true);             findfile(patch, filename);             t.start();         }          @override         public void run() {             bufferedreader reader;             while (!thread.currentthread().isinterrupted()) {                 try {                     lock.readlock().lock();                     reader = new bufferedreader(new inputstreamreader(new fileinputstream(file)));                     if (reader.readline().charat(0) == 'a') {                         lock.readlock().unlock();                         lock.writelock().lock();                         system.out.println("b try took file: " + file.getname());                         write();                         lock.writelock().unlock();                     } else {                         system.out.println("b took file: " + file.getname());                     }                 } catch (filenotfoundexception e)                 {                     system.out.println("file read b" + e.tostring());                 }                 catch (ioexception e)                 {                     system.out.println("file read b"+e.tostring());                 }             }         }           private void write() {             try {                 printwriter printwriter = new printwriter(file);                 printwriter.println("b took file: " + file.getname());                 system.out.println("b took file: " + file.getname());                 printwriter.close();             } catch (exception e) {                 system.out.println("file write b");             }         }          public file findfile(string patch,string filename) {             file file= null;             file folder = new file(patch);             file[] listoffiles = folder.listfiles();             bufferedreader reader;             (int = 0; < listoffiles.length; i++) {                 file = listoffiles[i];                 if (file.getname().equals(filename + ".txt")) {                     break;                 }             }             return file;         }     }