Call and receive output from Python script in Java

Question

What s the easiest way to execute a Python script from Java  and receive the output of that script  I ve looked for different libraries like Jepp or Jython  but most appear out of date  Another problem with the libraries is that I need to be able to easily include a library with the source code  though I don t need to source for the library itself  if I use a library    Because of this  would the easiest most effective way be to simply do something like call the script with runtime exec  and then somehow capture printed output  Or  even though it would be very painful for me  I could also just have the Python script output to a temporary text file  then read the file in Java   Note  the actual communication between Java and Python is not a requirement of the problem I am trying to solve  This is  however  the only way I can think of to easily perform what needs to be done

User · Answer

You can try using groovy  It runs on the JVM and it comes with great support for running external processes and extracting the output   http   groovy codehaus org Executing External Processes From Groovy  You can see in this code taken from the same link how groovy makes it easy to get the status of the process   println  return code     proc exitValue     println  stderr    proc err text   println  stdout    proc in text       out  from the external program is  in  for groovy

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ProcessBuilder is very easy to use   ProcessBuilder pb   new ProcessBuilder  python   Your python file     Command line arguments if any   Process p   pb start      This should call python  Refer to the process approach here for full example    https   bytes com topic python insights 949995-three-ways-run-python-programs-java

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Jep is anther option  It embeds CPython in Java through JNI   import jep Jep            try Jep jep   new Jep false             jep eval  s    hello world             jep eval  print s             jep eval  a   1   2            Long a    Long  jep getValue  a

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I ve looked for different libraries like Jepp or Jython  but most seem to be very out of date    Jython is not  a library   it s an implementation of the Python language on top of the Java Virtual Machine  It is definitely not out of date  the most recent release was Feb  24 of this year  It implements Python 2 5  which means you will be missing a couple of more recent features  but it is honestly not much different from 2 7      Note  the actual communication between Java and Python is not a requirement of the aforementioned assignment  so this isn t doing my homework for me  This is  however  the only way I can think of to easily perform what needs to be done     This seems extremely unlikely for a school assignment  Please tell us more about what you re really trying to do  Usually  school assignments specify exactly what languages you ll be using for what  and I ve never heard of one that involved more than one language at all  If it did  they d tell you if you needed to set up this kind of communication  and how they intended you to do it

User · Answer

Jython approach Java is supposed to be platform independent  and to call a native application  like python  isn t very platform independent  There is a version of Python  Jython  which is written in Java  which allow us to embed Python in our Java programs  As usually  when you are going to use external libraries  one hurdle is to compile and to run it correctly  therefore we go through the process of building and running a simple Java program with Jython  We start by getting hold of jython jar file  https   www jython org download html I copied jython-2 5 3 jar to the directory where my Java program was going to be  Then I typed in the following program  which do the same as the previous two  take two numbers  sends them to python  which adds them  then python returns it back to our Java program  where the number is outputted to the screen  import org python util PythonInterpreter   import org python core      class test3      public static void main String a              PythonInterpreter python   new PythonInterpreter             int number1   10          int number2   32          python set  quot number1 quot   new PyInteger number1            python set  quot number2 quot   new PyInteger number2            python exec  quot number3   number1 number2 quot            PyObject number3   python get  quot number3 quot            System out println  quot val    quot  number3 toString              I call this file  quot test3 java quot   save it  and do the following to compile it  javac -classpath jython-2 5 3 jar test3 java  The next step is to try to run it  which I do the following way  java -classpath jython-2 5 3 jar   test3  Now  this allows us to use Python from Java  in a platform independent manner  It is kind of slow  Still  it s kind of cool  that it is a Python interpreter written in Java

User · Answer

The best way to achieve would be to use Apache Commons Exec as I use it for production without problems even for Java 8 environment because of the fact that it lets you execute any external process  including python  bash etc  in synchronous and asynchronous way by using watchdogs    CommandLine cmdLine   new CommandLine  python     cmdLine addArgument   my python script script py     DefaultExecuteResultHandler resultHandler   new DefaultExecuteResultHandler      ExecuteWatchdog watchdog   new ExecuteWatchdog 60 1000    Executor executor   new DefaultExecutor     executor setExitValue 1    executor setWatchdog watchdog    executor execute cmdLine  resultHandler        some time later the result handler callback was invoked so we     can safely request the exit value  resultHandler waitFor      Complete source code for a small but complete POC is shared here that addresses another concern in this post    https   github com raohammad externalprocessfromjava git

User · Answer

Not sure if I understand your question correctly  but provided that you can call the Python executable from the console and just want to capture its output in Java  you can use the exec   method in the Java Runtime class   Process p   Runtime getRuntime   exec  python yourapp py      You can read up on how to actually read the output here   http   www devdaily com java edu pj pj010016  There is also an Apache library  the Apache exec project  that can help you with this  You can read more about it here   http   www devdaily com java java-exec-processbuilder-process-1  http   commons apache org exec

User · Answer

First I would recommend to use ProcessBuilder   since 1 5     Simple usage is described here  https   stackoverflow com a 14483787  For more complex example refer to   http   www javaworld com article 2071275 core-java when-runtime-exec---won-t html  I ve encountered problem when launching Python script from Java  script was producing too much output to standard out and everything went bad

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I met the same problem before  also read the answers here  but doesn t found any satisfy solution can balance the compatibility  performance and well format output  the Jython can t work with extend C packages and slower than CPython  So finally I decided to invent the wheel myself  it took my 5 nights  I hope it can help you too  jpserve https   github com johnhuang-cn jpserve     JPserve provides a simple way to call Python and exchange the result by well format JSON  few performance loss  The following is the sample code   At first  start jpserve on Python side   gt  gt  gt  from jpserve jpserve import JPServe  gt  gt  gt  serve   JPServe   localhost   8888    gt  gt  gt  serve start    INFO JPServe JPServe starting    INFO JPServe JPServe listening in localhost 8888   Then call Python from JAVA side   PyServeContext init  localhost   8888   PyExecutor executor   PyServeContext getExecutor    script    a   2 n         b   3 n          result    a   b    PyResult rs   executor exec script   System out println  Result      rs getResult      --- Result  6

User · Answer

You can include the Jython library in your Java Project  You can download the source code from  the Jython project itself   Jython does offers support for JSR-223 which basically lets you run a Python script from Java   You can use a ScriptContext to configure where you want to send your output of the execution   For instance  let s suppose you have the following Python script in a file named numbers py   for i in range 1 10       print i    So  you can run it from Java as follows   public static void main String   args  throws ScriptException  IOException        StringWriter writer   new StringWriter      ouput will be stored here      ScriptEngineManager manager   new ScriptEngineManager        ScriptContext context   new SimpleScriptContext         context setWriter writer     configures output redirection     ScriptEngine engine   manager getEngineByName  python        engine eval new FileReader  numbers py    context       System out println writer toString          And the output will be   1 2 3 4 5 6 7 8 9   As long as your Python script is compatible with Python 2 5 you will not have any problems running this with Jython

[java] Call and receive output from Python script in Java?

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