QuickStart Guide

Quick start quide to using JPype. This quide will show a series of simple examples with the corresponding commands in both Java and Python for using JPype. The JPype User Guide and API Reference have addition details on the use of the JPype module.

JPype uses two factory classes (JArray and JClass) to produce class wrappers which can be used to create all Java objects. These serve as both the base class for the corresponding hierarchy and as the factory to produce new wrappers. Casting operators are used to construct specify types of Java types (JObject, JString, JBoolean, JByte, JChar, JShort, JInt, JLong, JFloat, JDouble). Two special classes serve as the base classes for exceptions (JException) and interfaces (JInterface). There are a small number of support methods to help in controlling the JVM. Last, there are a few annotations used to create customized wrappers.

For the purpose of this guide, we will assume that the following classes were defined in Java. We will also assume the reader knows enough Java and Python to be dangerous.

package org.pkg;

publc class BassClass
{
   public callMember(int i)
   {}
}

public class MyClass extends BaseClass
{
   final public static int CONST_FIELD = 1;
   public static int staticField = 1;
   public int memberField = 2;
   int internalField =3;

   public MyClass() {}
   public MyClass(int i) {}

   public static void callStatic(int i) {}
   public void callMember(int i) {}

   // Python name conflict
   public void pass() {}

   public void throwsException throws java.lang.Exception {}

   // Overloaded methods
   public call(int i) {}
   public call(double d) {}
}

Starting JPype

The hardest thing about using JPype is getting the jars loaded into the JVM. Java is curiously unfriendly about reporting problems when it is unable to find a jar. Instead, it will be reported as an ImportError in python. These patterns will help debug problems regarding jar loading.

Once the JVM is started Java packages that are within a top level domain (TLD) are exposed as python modules allowing Java to be treated as part of python.

Description

Java

Python

Start Java Virtual Machine (JVM)

# Import module
import jpype

# Enable Java imports
import jpype.imports

# Pull in types
from jpype.types import *

# Launch the JVM
jpype.startJVM()

Import default Java namespace 1

import java.lang

Add a set of jars from a directory 2

jpype.addClassPath("/my/path/*")

Add a specific jar to the classpath 2

jpype.addClassPath('/my/path/myJar.jar')

Print JVM CLASSPATH 3

from java.lang import System
print(System.getProperty("java.class.path"))
1

All java.lang.* classes are available.

2(1,2)

Must happen prior to starting the JVM

3

After JVM is started

Classes/Objects

Java classes are presented whereever possible exactly like Python classes. The only major difference is that Java classes and objects are closed and cannot be modified. As Java is strongly typed, casting operators are used to select specific overloads when calling methods. Classes are either imported using as a module or loaded with the JClass factory.

Description

Java

Python

Import a class 4

import org.pkg.MyClass
from org.pkg import MyClass

Import a class and rename 4

from org.pkg import MyClass as OurClass

Import multiple classes from a package 5

from org.pkg import MyClass, AnotherClass

Import a java package for long name access 6

import org.pkg

Import a class static 7

import org.pkg.MyClass.CONST_FIELD
from org.pkg.MyClass import CONST_FIELD

Import a class without tld 8

import zippy.NonStandard
NonStandard = JClass('zippy.NonStandard')

Construct an object

MyClass myObject = new MyClass(1);
myObject = MyClass(1)

Constructing a cless with full class name

import org.pkg
myObject = org.pkg.MyClass(args)

Get a static field

int var = MyClass.staticField;
var = MyClass.staticField

Get a member field

int var = myObject.memberField;
var = myObject.memberField

Set a static field 9

MyClass.staticField = 2;
MyClass.staticField = 2

Set a member field 9

myObject.memberField = 2;
myObject.memberField = 2

Call a static method

MyClass.callStatic(1);
MyClass.callStatic(1)

Call a member method

myObject.callMember(1);
myObject.callMember(1)

Access member with python naming conflict 10

myObject.pass()
myObject.pass_()

Checking inheritance

if (obj instanceof MyClass) {...}
if (isinstance(obj, MyClass): ...

Checking if Java class wrapper

if (isinstance(obj, JClass): ...

Checking if Java object wrapper

if (isinstance(obj, JObject): ...

Casting to a specific type

BaseClass b = (BaseClass)myObject;
b = JObject(myObject, BaseClass)
4(1,2)

This will report an error if the class is not found.

5

This will report an error if the classes are not found

6

Does not report errors if the package is invalid

7

Constants, static fields, and static methods can be imported.

8

JClass loads any class by name including inner classes.

9(1,2)

Produces error for final fields

10

Underscore is added during wrapping.

Exceptions

Java exceptions extend from python exceptions and can be dealt with no different that Python native exceptions. JException serves as the base class for all Java exceptions.

Description

Java

Python

Catch an exception

try {
   myObject.throwsException();
} catch (java.lang.Exception ex)
{ ... }
try:
    myObject.throwsException()
except java.lang.Exception as ex:
    ...

Throw an exception to Java

throw new java.lang.Exception(
        "Problem");
raise java.lang.Exception(
        "Problem")

Checking if Java exception wrapper

if (isinstance(obj, JException): ...

Closeable items

try (InputStream is
  = Files.newInputStream(file))
{ ... }
with Files.newInputStream(file) as is:
   ...

Primitives

Most python primitives directly map into Java primitives. However, python does not have the same primitive types, thus sometimes it is necessary to cast to a specific Java primitive type especially if there are Java overloads that would otherwise be in conflict. Each of the Java types are exposed in JPype (JBoolean, JByte, JChar, JShort, JInt, JLong, JFloat, JDouble).

Python int is equivalent to Java long.

Description

Java

Python

Casting to hit an overload 11

myObject.call((int)v);
myObject.call(JInt(v))

Create a primitive array

int[] array = new int[5]
array = JArray(JInt)(5)

Create an initialized primitive array 12

int[] array = new int[]{1,2,3}
array = JArray(JInt)([1,2,3])

Put a specific primitive type on a list

List<Integer> myList
  = new ArrayList<>();
myList.add(1);
from java.util import ArrayList
myList = ArrayList()
myList.add(JInt(1))

Boxing a primitive 13

Integer boxed = 1;
boxed = JObject(JInt(1))
11

JInt acts as a casting operator

12

list, sequences, or np.array can be used to initialize.

13

JInt specifies the prmitive type. JObject boxes the primitive.

Strings

Java strings are similar to python strings. They are both immutable and produce a new string when altered. Most operations can use Java strings in place of python strings, with minor exceptions as python strings are not completely duck typed. When comparing or using as dictionary keys JString should be converted to python.

Description

Java

Python

Create a Java string 14

String javaStr = new String("foo");
myStr = JString("foo")

Create a Java string from bytes 15

byte[] b;
String javaStr = new String(b, "UTF-8");
b= b'foo'
myStr = JString(b, "UTF-8")

Converting Java string

str(javaStr)

Comparing Python and Java strings 16

str(javaStr) == pyString

Comparing Java strings

javaStr.equals("foo")
javaStr == "foo"

Checking if java string

if (isinstance(obj, JString): ...
14

JString constructs a java.lang.String

15

All java.lang.String constuctors work.

16

str() converts the object for comparison

Arrays

Arrays are create using JArray class factory. They operate like python lists, but they are fixed in size.

Description

Java

Python

Create a single dimension array

MyClass[] array = new MyClass[5];
array = JArray(MyClass)(5)

Create a multi dimension array

MyClass[][] array2 = new MyClass[5][];
array2 = JArray(MyClass, 2)(5)

Access an element

array[0] = new MyClass()
array[0] = MyClass()

Size of an array

array.length
len(array)

Convert to python list

pylist = list(array)

Iterate elements

for (MyClass element: array)
{...}
for element in array:
  ...

Checking if java array wrapper

if (isinstance(obj, JArray): ...

Collections

Java standard containers are available and are overloaded with python syntax where possible to operate in a similar fashion to python objects. It is not currently possible to specify the template types for generic containers, but that will be introduced in Java 9.

Description

Java

Python

Import list type

import java.util.ArrayList;
from java.util import ArrayList

Construct a list

List<Integer> myList=new ArrayList<>();
myList=ArrayList()

Get length of list

int sz = myList.size();
sz = len(myList)

Get list item

Integer i = myList.get(0)
i = myList[0]

Set list item 17

myList.set(0, 1)
myList[0]=Jint(1)

Iterate list elements

for (Integer element: myList)
{...}
for element in myList:
  ...

Import map type

import java.util.HashMap;
from java.util import HashMap

Construct a map

Map<String,Integer> myMap=new HashMap<>();
myMap=HashMap()

Get length of map

int sz = myMap.size();
sz = len(myMap)

Get map item

Integer i = myMap.get("foo")
i = myMap["foo"]

Set map item 17

myMap.set("foo", 1)
myMap["foo"]=Jint(1)

Iterate map entries

for (Map.Entry<String,Integer> e
  : myMap.entrySet())
  {...}
for e in myMap.entrySet():
  ...
17(1,2)

Casting is required to box primitives to the correct type.

Reflection

For operations that are outside the scope of the JPype syntax, Using Java reflection, any Java operation include calling a specific overload or even accessing private methods and fields.

Description

Java

Python

Access Java reflection class

MyClass.class
MyClass.class_

Access a private field by name

cls = myObject.class_
field = cls.getDeclaredField(
    "internalField")
field.setAccessible(True)
field.get()

Accessing a specific overload 18

cls = MyClass.class_
cls.getDeclaredMethod("call", JInt)
cls.invoke(myObject, JInt(1))

Convert a java.lang.Class into python wrapper 19

# Something returned a java.lang.Class
MyClassJava = getClassMethod()

# Convert to it to Python
MyClass = JClass(myClassJava)

Load a class with a external class loader

ClassLoader cl
  = new ExternalClassLoader();
Class cls
  = Class.forName("External",
                  True, cl)
cl = ExternalClassLoader()
cls = JClass("External", loader=cl)

Accessing base method implementation

from org.pkg import \
        BaseClass, MyClass
myObject = MyClass(1)
BaseClass.callMember(myObject, 2)
18

types must be exactly specified.

19

Rarely required unless the class was supplied external such as generics.

Implements and Extension

JPype can implement a Java interface by annotating a python class. Each method that is required must be implemented.

JPype does not support extending a class directly in python. Where it is necessary to exend a Java class, it is required to create a Java extension with an interface for each methods that are to be accessed from python. For some deployments this may be be an option. If that is the case, the JPype inline compiler can be used to create the dynamic class on the fly.

Description

Java

Python

Implement an interface

public class PyImpl
  implements MyInterface
{
  public void call()
  {...}
}
@JImplements(MyInterface)
class PyImpl(object):
    @JOverride
    def call(self):
      pass

Extending classes 20

None

Lambdas 20

None

20(1,2)

Support for use of python function as Java 8 lambda is WIP.

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