Java QuickStart Guide

This is a quick start guide to using JPype with Java. This guide will show a series of snippets with the corresponding commands in both Java and Python for using JPype. The JPype User Guide and API Reference have additional 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. Lastly, 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 BaseClass
{
   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 with 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()
Start Java Virtual Machine (JVM) with a classpath   
# Launch the JVM
jpype.startJVM(classpath = ['jars/*'])
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 wherever possible similar to 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 a module, loaded using JPackage 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 class 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 [11] 
BaseClass b = (BaseClass)myObject;
 
[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) This produces an error for final fields.
[10]Underscore is added during wrapping.
[11]Matmul(@) is used as the casting operator.

Exceptions

Java exceptions extend from Python exceptions and can be dealt with in the same way as 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, and it is necessary to cast to a specific Java primitive type whenever 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).

Description Java Python
Casting to hit an overload [12] 
myObject.call((int)v);

myObject.call(JInt(v))
Create a primitive array 
int[] array = new int[5]

array = JInt[5]
Create a rectangular primitive array 
int[][] array = new int[5][10]

array = JInt[5, 10]
Create an array of arrays 
int[][] array = new int[5][]

array = JInt[5, :]
Create an initialized primitive array [13] 
int[] array = new int[]{1,2,3}

array = JInt[:]([1,2,3])
Create an initialized boxed array [13] 
Integer[] array = new Integer[]{1,2,3}

array = java.lang.Integer[:]([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 [14] 
Integer boxed = 1;

boxed = JObject(JInt(1))
[12]JInt acts as a casting operator
[13](1, 2) list, sequences, or np.array can be used to initialize.
[14]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, all JString objects should be converted to Python.

Description Java Python
Create a Java string [15] 
String javaStr = new String("foo");

myStr = JString("foo")
Create a Java string from bytes [16] 
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 [17]   
str(javaStr) == pyString
Comparing Java strings 
javaStr.equals("foo")

javaStr == "foo"
Checking if Java string   
if (isinstance(obj, JString): ...
[15]JString constructs a java.lang.String
[16]All java.lang.String constuctors work.
[17]str() converts the object for comparison

Arrays

Arrays are create using the 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 = MyClass[5]
Create a multi dimension array (old) 
MyClass[][] array2 = new MyClass[5][];

array2 = JArray(MyClass, 2)(5)
Create a multi dimension array (new) 
MyClass[][] array2 = new MyClass[5][];

array2 = MyClass[5,:]
Access an element 
array[0] = new MyClass()

array[0] = MyClass()
Size of an array 
array.length

len(array)
Get last element 
MyClass a = array[array.length];

a = array[-1]
Slice an array [18]   
a = array[2:5]
Clone an array 
MyClass[] a = array.clone();

a = array.clone()
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): ...
[18]A Slice is a view and changes will be reflected on original. Slices passed to Java will clone.

Collections

Java standard containers are available and are overloaded with Python syntax where possible to operate in a similar fashion to Python objects.

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 [19] 
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 [19] 
myMap.set("foo", 1)

myMap["foo"] = Jint(1)
Iterate map entries 
for (Map.Entry<String,Integer> e
  : myMap.entrySet())
  {...}

for e in myMap.entrySet():
  ...
[19](1, 2) Casting is required to box primitives to the correct type.

Reflection

Java reflection can be used to access operations that are outside the scope of the JPype syntax. This includes 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 [20]   
cls = myObject.class_
field = cls.getDeclaredField(
    "internalField")
field.setAccessible(True)
field.get()
Accessing a specific overload [21]   
cls = MyClass.class_
cls.getDeclaredMethod("call", JInt)
cls.invoke(myObject, JInt(1))
Convert a java.lang.Class into Python wrapper [22]   
# 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)
[20]This is prohibited after Java 8
[21]types must be exactly specified.
[22]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.

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 [23]   None
Lambdas [24] 
DoubleUnaryOperator u = (p->p*2);
 
[23]Support for use of Python function as Java 8 lambda is WIP.
[24]Any Java functional interface can take a lambda or callable.

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