Calling Python in Java

Question

I am wondering if it is possible to call python functions from java code using jython  or is it only for calling java code from python

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Several of the answers mention that you can use JNI or JNA to access cpython but I would not recommend starting from scratch because there are already open source libraries for accessing cpython from java  For example    JEP JPY

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Jython  Python for the Java Platform - http   www jython org index html  You can easily call python functions from Java code with Jython  That is as long as your python code itself runs under jython  i e  doesn t use some c-extensions that aren t supported   If that works for you  it s certainly the simplest solution you can get  Otherwise you can use org python util PythonInterpreter from the new Java6 interpreter support   A simple example from the top of my head - but should work I hope   no error checking done for brevity   PythonInterpreter interpreter   new PythonInterpreter    interpreter exec  import sys nsys path append  pathToModules if they are not there by default   nimport yourModule       execute a function that takes a string and returns a string PyObject someFunc   interpreter get  funcName    PyObject result   someFunc   call   new PyString  Test      String realResult    String  result   tojava   String class

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Depending on your requirements  options like XML-RPC could be useful  which can be used to remotely call functions virtually in any language supporting the protocol

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Jython has some limitations      There are a number of differences  First  Jython programs cannot use CPython    extension modules written in C  These modules usually have files with the   extension  so   pyd or  dll  If you want to use such a module  you should look    for an equivalent written in pure Python or Java  Although it is technically    feasible to support such extensions - IronPython does so - there are no plans    to do so in Jython    Distributing my Python scripts as JAR files with Jython   you can simply call python scripts  or bash or Perl scripts  from Java using Runtime or ProcessBuilder and pass output back to Java   Running a bash shell script in java   Running Command Line in Java  java runtime getruntime   getting output from executing a command line program

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You can call any language from java using Java Native Interface

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Here a library that lets you write your python scripts once and decide which integration method  Jython  CPython PyPy via Jep and Py4j  to use at runtime    https   github com subes invesdwin-context-python   Since each method has its own benefits drawbacks as explained in the link

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It depends on what do you mean by python functions  if they were written in cpython you can not directly call them you will have to use JNI  but if they were written in Jython you can easily call them from java  as jython ultimately generates java byte code   Now when I say written in cpython or jython it doesn t make much sense because python is python and most code will run on both implementations unless you are using specific libraries which relies on cpython or java   see here how to use Python interpreter in Java

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GraalVM is a good choice  I ve done Java Javascript combination with GraalVM for microservice design  Java with Javascript reflection   They recently added support for python  I d give it a try especially with how big its community has grown over the years

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Hey I thought I would enter my answer to this even though its late  I think there are some important things to consider first with how strong you wish to have the linking between java and python  Firstly  Do you only want to call functions or do you actually want python code to change the data in your java objects  This is very important  If you only want to call some python code with or without arguments  then that is not very difficult  If your arguments are primitives it makes it even more easy  However if you want to have java class implement member functions in python  which change the data of the java object  then this is not so easy or straight forward  Secondly are we talking cpython or will jython do  I would say cpython is where its at  I would advocate this is why python is so kool  Having such high abstractions however access to c c   when needed  Imagine if you could have that in java  This question is not even worth asking if jython is ok because then it is easy anyway  So I have played with the following methods  and listed them from easy to difficult  Java to Jython Advantages  Trivially easy  Have actual references to java objects Disadvantages  No CPython  Extremely Slow  Jython from java is so easy  and if this is really enough then great  However it is very slow and no cpython  Is life worth living without cpython I don t think so  You can easily have python code implementing your member functions for you java objects  Java to Jython to CPython via Pyro Pyro is the remote object module for python  You have some object on a cpython interpreter  and you can send it objects which are transferred via serialization and it can also return objects via this method  Note that if you send a serialized python object from jython and then call some functions which change the data in its members  then you will not see those changes in java  You just need to remember to send back the data which you want from pyro  This I believe is the easiest way to get to cpython  You do not need any jni or jna or swig or      You don t need to know any c  or c    kool huh  Advantages  Access to cpython  not as difficult as following methods Disadvantages  Cannot change the member data of java objects directly from python  Is somewhat indirect   jython is middle man   Java to C C   via JNI JNA SWIG to Python via Embedded interpreter  maybe using BOOST Libraries   OMG this method is not for the faint of heart  And I can tell you it has taken me very long to achieve this in with a decent method  Main reason you would want to do this is so that you can run cpython code which as full rein over you java object  There are major major things to consider before deciding to try and bread java  which is like a chimp  with python  which is like a horse   Firstly if you crash the interpreter that s lights out for you program  And don t get me started on concurrency issues  In addition  there is allot allot of boiler  I believe I have found the best configuration to minimize this boiler but still it is allot  So how to go about this  Consider that C   is your middle man  your objects are actually c   objects  Good that you know that now  Just write your object as if your program as in cpp not java  with the data you want to access from both worlds  Then you can use the wrapper generator called swig  http   www swig org Doc1 3 Java html  to make this accessible to java and compile a dll which you call System load dll name here  in java  Get this working first  then move on to the hard part  To get to python you need to embed an interpreter  Firstly I suggest doing some hello interpreter programs or this tutorial Embedding python in C C  Once you have that working  its time to make the horse and the monkey dance  You can send you c   object to python via  boost  3    I know I have not given you the fish  merely told you where to find the fish  Some pointers to note for this when compiling  When you compile boost you will need to compile a shared library  And you need to include and link to the stuff you need from jdk  ie jawt lib  jvm lib   you will also need the client jvm dll in your path when launching the application  As well as the python27 lib or whatever and the boost python-vc100-mt-1 55 lib  Then include Python include  jdk include  boost and only use shared libraries  dlls  otherwise boost has a teary  And yeah full on I know  There are so many ways in which this can go sour  So make sure you get each thing done block by block  Then put them together

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It s not smart to have python code inside java  Wrap your python code with flask or other web framework to make it as a microservice  Make your java program able to call this microservice  e g  via REST    Beleive me  this is much simple and will save you tons of issues  And the codes are loosely coupled so they are scalable   Updated on Mar 24th 2020  According to  stx s comment  the above approach is not suitable for massive data transfer between client and server   Here is another approach I recommended   Connecting Python and Java with Rust C C   also ok   https   medium com  shmulikamar https-medium-com-shmulikamar-connecting-python-and-java-with-rust-11c256a1dfb0

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This gives a pretty good overview over the current options  Some of which are named in other answers  Jython is not usable until they decide to implement Python 3 x and many of the other projects are coming form the python side and want to access java  But there are a few options still  to name something which has not been named yet  gRPC

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