process waitFor never returns

Question

Process process   Runtime getRuntime   exec  tasklist    BufferedReader reader        new BufferedReader new InputStreamReader process getInputStream      process waitFor

User · Answer

I would like to add something to the previous answers but since I don't have the rep to comment, I will just add an answer. This is directed towards android users which are programming in Java.

Per the post from RollingBoy, this code almost worked for me:

Process process = Runtime.getRuntime().exec("tasklist");
BufferedReader reader =
new BufferedReader(new InputStreamReader(process.getInputStream()));
while ((reader.readLine()) != null) {}
process.waitFor();

In my case, the waitFor() was not releasing because I was executing a statement with no return ("ip adddr flush eth0"). An easy way to fix this is to simply ensure you always return something in your statement. For me, that meant executing the following: "ip adddr flush eth0 && echo done". You can read the buffer all day, but if there is nothing ever returned, your thread will never release its wait.

Hope that helps someone!

User · Answer

I think I observed a similar problem  some processes started  seemed to run successfully but never completed  The function waitFor   was waiting forever except if I killed the process in Task Manager  However  everything worked well in cases the length of the command line was 127 characters or shorter   If long file names are inevitable you may want to use environmental variables  which may allow you keeping the command line string short   You can generate a batch file  using FileWriter  in which you set your environmental variables before calling the program you actually want to run  The content of such a batch could look like       set INPUTFILE  C  Directory 0 Subdirectory 1 AnyFileName      set OUTPUTFILE  C  Directory 2 Subdirectory 3 AnotherFileName      set MYPROG  C  Directory 4 Subdirectory 5 ExecutableFileName exe       MYPROG   INPUTFILE   OUTPUTFILE    Last step is running this batch file using Runtime

User · Answer

As others have mentioned you have to consume stderr and stdout   Compared to the other answers  since Java 1 7 it is even more easy  You do not have to create threads yourself anymore to read stderr and stdout   Just use the ProcessBuilder and use the methods redirectOutput in combination with either redirectError or redirectErrorStream   String directory     working dir   File out   new File          File to write stdout to File err   new File          File to write stderr to ProcessBuilder builder   new ProcessBuilder    builder directory new File directory    builder command command   builder redirectOutput out      Redirect stdout to file if out    err       builder redirectErrorStream true      Combine stderr into stdout   else      builder redirectError err      Redirect stderr to file   Process process   builder start

User · Answer

Here is a method that works for me  NOTE  There is some code within this method that may not apply to you  so try and ignore it   For example  logStandardOut       git-bash  etc    private String exeShellCommand String doCommand  String inDir  boolean ignoreErrors    logStandardOut   gt   s   doCommand    ProcessBuilder builder   new ProcessBuilder    StringBuilder stdOut   new StringBuilder    StringBuilder stdErr   new StringBuilder     boolean isWindows   System getProperty  os name   toLowerCase   startsWith  windows    if  isWindows      String gitBashPathForWindows    C   Program Files  Git  bin  bash     builder command gitBashPathForWindows   -c   doCommand     else     builder command  bash    -c   doCommand        Do we need to change dirs  if  inDir    null      builder directory new File inDir         Execute it Process process   null  BufferedReader brStdOut  BufferedReader brStdErr  try       Start the command line process   process   builder start         This hangs on a large file      https   stackoverflow com questions 5483830 process-waitfor-never-returns     exitCode   process waitFor         This will have both StdIn and StdErr   brStdOut   new BufferedReader new InputStreamReader process getInputStream        brStdErr   new BufferedReader new InputStreamReader process getErrorStream           Get the process output   String line   null    String newLineCharacter   System getProperty  line separator       while  process isAlive            Read the stdOut     while   line   brStdOut readLine       null          stdOut append line   newLineCharacter                Read the stdErr     while   line   brStdErr readLine       null          stdErr append line   newLineCharacter                Nothing else to read  lets pause for a bit before trying again     process waitFor 100  TimeUnit MILLISECONDS            Read anything left  after the process exited   while   line   brStdOut readLine       null        stdOut append line   newLineCharacter            Read anything left  after the process exited   while   line   brStdErr readLine       null        stdErr append line   newLineCharacter            cleanup   if  brStdOut    null        brStdOut close           if  brStdErr    null        brStdOut close             Log non-zero exit values   if   ignoreErrors  amp  amp  process exitValue      0        String exMsg   String format   s nprocess exitValue  s   stdErr  process exitValue         throw new ExecuteCommandException exMsg          catch  ExecuteCommandException e      throw e    catch  Exception e      throw new ExecuteCommandException stdErr toString    e     finally       Log the results   logStandardOut stdOut toString       logStandardError stdErr toString        return stdOut toString

User · Answer

You should try consume output and error in the same while      private void runCMD String CMD  throws IOException  InterruptedException       System out println  Standard output      CMD       Process process   Runtime getRuntime   exec CMD           Get input streams     BufferedReader stdInput   new BufferedReader new InputStreamReader process getInputStream          BufferedReader stdError   new BufferedReader new InputStreamReader process getErrorStream          String line           String newLineCharacter   System getProperty  line separator         boolean isOutReady   false      boolean isErrorReady   false      boolean isProcessAlive   false       boolean isErrorOut   true      boolean isErrorError   true        System out println  Read command         while  process isAlive                Read the stdOut          do               isOutReady   stdInput ready                  System out println  OUT READY     isOutReady               isErrorOut   true              isErrorError   true               if  isOutReady                    line   stdInput readLine                    isErrorOut   false                  System out println                                                                                           line   newLineCharacter                             isErrorReady   stdError ready                  System out println  ERROR READY     isErrorReady               if  isErrorReady                    line   stdError readLine                    isErrorError   false                  System out println  ERROR                                                                                          line   newLineCharacter                              isProcessAlive   process isAlive                  System out println  Process Alive     isProcessAlive               if   isProcessAlive                    System out println                                                                                         Process DIE     line   newLineCharacter                   line   null                  isErrorError   false                  process waitFor 1000  TimeUnit MILLISECONDS                            while  line    null              Nothing else to read  lets pause for a bit before trying again         System out println  PROCESS WAIT FOR            process waitFor 100  TimeUnit MILLISECONDS             System out println  Command finished

User · Answer

There are many reasons that waitFor   doesn t return  But it usually boils down to the fact that the executed command doesn t quit  This  again  can have many reasons  One common reason is that the process produces some output and you don t read from the appropriate streams  This means that the process is blocked as soon as the buffer is full and waits for your process to continue reading  Your process in turn waits for the other process to finish  which it won t because it waits for your process        This is a classical deadlock situation  You need to continually read from the processes input stream to ensure that it doesn t block  There s a nice article that explains all the pitfalls of Runtime exec   and shows ways around them called  quot When Runtime exec   won t quot   yes  the article is from 2000  but the content still applies

User · Answer

There are several possibilities    You haven t consumed all the output on the process s stdout  You haven t consumed all the output on the process s stderr  The process is waiting for input from you and you haven t provided it  or you haven t closed the process s stdin  The process is spinning in a hard loop

User · Answer

Also from Java doc      java lang       Class Process      Because some native platforms only provide limited buffer size for standard input and   output streams  failure to promptly write the input stream or read the output stream of   the subprocess may cause the subprocess to block  and even deadlock       Fail to clear the buffer of input stream  which pipes to the output stream of subprocess     from Process may lead to a subprocess blocking    Try this   Process process   Runtime getRuntime   exec  tasklist    BufferedReader reader   new BufferedReader new InputStreamReader process getInputStream      while   reader readLine       null     process waitFor

User · Answer

For the same reason you can also use inheritIO   to map Java console with external app console like   ProcessBuilder pb   new ProcessBuilder appPath  arguments    pb directory new File appFile getParent      pb inheritIO     Process process   pb start    int success   process waitFor

User · Answer

It appears you are not reading the output before waiting for it to finish   This is fine only if the output doesn t fill the buffer   If it does  it will wait until you read the output  catch-22   Perhaps you have some errors which you are not reading   This would case the application to stop and waitFor to wait forever   A simple way around this is to re-direct the errors to the regular output   ProcessBuilder pb   new ProcessBuilder  tasklist    pb redirectErrorStream true   Process process   pb start    BufferedReader reader   new BufferedReader new InputStreamReader process getInputStream      String line  while   line   reader readLine       null      System out println  tasklist      line   process waitFor

User · Answer

Asynchronous reading of stream combined with avoiding Wait with a timeout will solve the problem  You can find a page explaining this here http   simplebasics net  net process-waitforexit-with-a-timeout-will-not-be-able-to-collect-the-output-message

[java] process.waitFor() never returns

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