This part of the documentation covers all the interfaces of Pyjnius.

Reflection classes

class jnius.JavaClass

Base for reflecting a Java class. The idea is to subclass this JavaClass, add few JavaMethod, JavaStaticMethod, JavaField, JavaStaticField, and you’re done.

You need to define at minimum the __javaclass__ attribute, and set the __metaclass__ to MetaJavaClass.

So the minimum class definition would look like:

from jnius import JavaClass, MetaJavaClass

class Stack(JavaClass):
    __javaclass__ = 'java/util/Stack'
    __metaclass__ = MetaJavaClass

Must be set to MetaJavaClass, otherwise, all the methods/fields declared will be not linked to the JavaClass.


Represent the Java class name, in the format ‘org/lang/Class’. (eg: ‘java/util/Stack’), not ‘org.lang.Class’.


If not set, we assume the default constructor to take no parameters. Otherwise, it can be a list of all possible signatures of the constructor. For example, a reflection of the String java class would look like:

class String(JavaClass):
    __javaclass__ == 'java/lang/String'
    __metaclass__ = MetaJavaClass
    __javaconstructor__ == (
        # ...
class jnius.JavaMethod

Reflection of a Java method.

__init__(signature, static=False)

Create a reflection of a Java method. The signature is in the JNI format. For example:

class Stack(JavaClass):
    __javaclass__ = 'java/util/Stack'
    __metaclass__ = MetaJavaClass

    peek = JavaMethod('()Ljava/lang/Object;')
    empty = JavaMethod('()Z')

The name associated to the method is automatically set from the declaration within the JavaClass itself.

The signature can be found with the javap -s. For example, if you want to fetch the signatures available for java.util.Stack:

$ javap -s java.util.Stack
Compiled from "Stack.java"
public class java.util.Stack extends java.util.Vector{
public java.util.Stack();
  Signature: ()V
public java.lang.Object push(java.lang.Object);
  Signature: (Ljava/lang/Object;)Ljava/lang/Object;
public synchronized java.lang.Object pop();
  Signature: ()Ljava/lang/Object;
public synchronized java.lang.Object peek();
  Signature: ()Ljava/lang/Object;
public boolean empty();
  Signature: ()Z
public synchronized int search(java.lang.Object);
  Signature: (Ljava/lang/Object;)I
class jnius.JavaStaticMethod

Reflection of a static Java method.

class jnius.JavaField

Reflection of a Java field.

__init__(signature, static=False)

Create a reflection of a Java field. The signature is in the JNI format. For example:

class System(JavaClass):
    __javaclass__ = 'java/lang/System'
    __metaclass__ = MetaJavaClass

    out = JavaField('()Ljava/io/InputStream;', static=True)

The name associated to the method is automatically set from the declaration within the JavaClass itself.

class jnius.JavaStaticField

Reflection of a static Java field

class jnius.JavaMultipleMethod

Reflection of a Java method that can be called from multiple signatures. For example, the method getBytes in the String class can be called from:

public byte[] getBytes(java.lang.String)
public byte[] getBytes(java.nio.charset.Charset)
public byte[] getBytes()

Let’s see how you could declare that method:

class String(JavaClass):
    __javaclass__ = 'java/lang/String'
    __metaclass__ = MetaJavaClass

    getBytes = JavaMultipleMethod([

Then, when you will try to access to this method, we’ll take the best method available according to the type of the arguments you’re using. Internally, we are calculating a “match” score for each available signature, and take the best one. Without going into the details, the score calculation look like:

  • a direct type match is +10
  • a indirect type match (like using a float for an int argument) is +5
  • object with unknown type (JavaObject) is +1
  • otherwise, it’s considered as an error case, and return -1

Reflection functions


Return a JavaClass that represent the class passed from name. The name must be written in the format: a.b.c, not a/b/c.

>>> from jnius import autoclass
>>> autoclass('java.lang.System')
<class 'jnius.java.lang.System'>

autoclass can also represent a nested Java class:

>>> autoclass('android.provider.Settings$Secure')
<class 'jnius.reflect.android.provider.Settings$Secure'>

Java class implementation in Python

class jnius.PythonJavaClass

Base for creating a Java class from a Python class. This allow to implement java interface completely in Python.

In reality, you’ll create a Python class that mimic the list of declared __javainterfaces__. When you’ll give an instance of this class to Java, Java will just accept it and call the interfaces methods as declared. Under the hood, we are catching the call, and redirecting to use your declared Python method.

Your class will act as a Proxy to the Java interfaces.

You need to define at minimum the __javainterfaces__ attribute, and declare java methods with the java_method() decorator.


Static methods and static fields are not supported

For example, you could implement the java/util/ListIterator interface in Python like that:

from jnius import PythonJavaClass, java_method

class PythonListIterator(PythonJavaClass):
    __javainterfaces__ = ['java/util/ListIterator']

    def __init__(self, collection, index=0):
        super(TestImplemIterator, self).__init__()
        self.collection = collection
        self.index = index

    def hasNext(self):
        return self.index < len(self.collection.data) - 1

    def next(self):
        obj = self.collection.data[self.index]
        self.index += 1
        return obj

    # etc...

List of the Java interfaces you want to proxify, in the format ‘org/lang/Class’. (eg: ‘java/util/Iterator’), not ‘org.lang.Class’.


Indicate which class loader to use: ‘system’ or ‘app’, default to ‘system’:

  • By default, we assume that you are going to implement a Java interface declared in the Java API. It will use the ‘system’ class loader.
  • On android, all the java interfaces that you ship within the APK are not accessible with the system class loader, but with the application thread class loader. So if you wish to implement a class from an interface you’ve done in your app, use ‘app’.
jnius.java_method(java_signature, name=None)

Decoration function to use with PythonJavaClass. The java_signature must match the wanted signature of the interface. The name of the method will be the name of the Python method by default. You can still force it, in case of multiple signature with the same Java method name.

For example:

class PythonListIterator(PythonJavaClass):
    __javainterfaces__ = ['java/util/ListIterator']

    def next(self):
        obj = self.collection.data[self.index]
        self.index += 1
        return obj

Another example with the same Java method name, but 2 differents signatures:

class TestImplem(PythonJavaClass):
    __javainterfaces__ = ['java/util/List']

    def listIterator(self):
        return PythonListIterator(self)

    def listIteratorWithIndex(self, index):
        return PythonListIterator(self, index)

Java signature format

Java signatures have a special format that could be difficult to understand at first. Let’s see in details. A signature is in the format:

(<argument1><argument2><...>)<return type>

All the types for any part of the signature can be one of:

  • L<java class>; = represent a Java object of the type <java class>
  • Z = represent a java/lang/Boolean;
  • B = represent a java/lang/Byte;
  • C = represent a java/lang/Character;
  • S = represent a java/lang/Short;
  • I = represent a java/lang/Integer;
  • J = represent a java/lang/Long;
  • F = represent a java/lang/Float;
  • D = represent a java/lang/Double;
  • V = represent void, available only for the return type

All the types can have the [ prefix to design an array. The return type can be V or empty.

A signature like:

-> argument 1 is an integer
-> argument 2 is a java.util.List object
-> the method doesn't return anything.

-> argument 1 is a Collection
-> argument 2 is an array of Object
-> nothing is returned

-> argument 1 is a Byte []
-> a boolean is returned

When you implement Java in Python, the signature of the Java method must match. Java provides a tool named javap to get the signature of any java class. For example:

$ javap -s java.util.Iterator
Compiled from "Iterator.java"
public interface java.util.Iterator{
public abstract boolean hasNext();
  Signature: ()Z
public abstract java.lang.Object next();
  Signature: ()Ljava/lang/Object;
public abstract void remove();
  Signature: ()V

JVM options and the class path

JVM options need to be set before import jnius is called, as they cannot be changed after the VM starts up. To this end, you can:

import jnius_config
jnius_config.add_options('-Xrs', '-Xmx4096')
jnius_config.set_classpath('.', '/usr/local/fem/plugins/*')
import jnius

If a classpath is set with these functions, it overrides any CLASSPATH environment variable. Multiple options or path entries should be supplied as multiple arguments to the add_ and set_ functions. If no classpath is provided and CLASSPATH is not set, the path defaults to ‘.’. This functionality is not available on Android.

Pyjnius and threads


Each time you create a native thread in Python and uses Pyjnius, any call to Pyjnius methods will force attachment of the native thread to the current JVM. But you must detach it before leaving the thread, and Pyjnius cannot do it for you.


import threading
import jnius

def run(...):
        # use pyjnius here

If you don’t, it will crash on dalvik and ART / Android:

D/dalvikvm(16696): threadid=12: thread exiting, not yet detached (count=0)
D/dalvikvm(16696): threadid=12: thread exiting, not yet detached (count=1)
E/dalvikvm(16696): threadid=12: native thread exited without detaching
E/dalvikvm(16696): VM aborting


W/art     (21168): Native thread exiting without having called DetachCurrentThread (maybe it's going to use a pthread_key_create destructor?): Thread[16,tid=21293,Native,Thread*=0x4c25c040,peer=0x677eaa70,"Thread-16219"]
F/art     (21168): art/runtime/thread.cc:903] Native thread exited without calling DetachCurrentThread: Thread[16,tid=21293,Native,Thread*=0x4c25c040,peer=0x677eaa70,"Thread-16219"]
F/art     (21168): art/runtime/runtime.cc:203] Runtime aborting...
F/art     (21168): art/runtime/runtime.cc:203] (Aborting thread was not attached to runtime!)
F/art     (21168): art/runtime/runtime.cc:203] Dumping all threads without appropriate locks held: thread list lock mutator lock
F/art     (21168): art/runtime/runtime.cc:203] All threads:
F/art     (21168): art/runtime/runtime.cc:203] DALVIK THREADS (16):