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

Reflection classes#

class jnius.JavaClass#

Base for reflecting a Java class, allowing access to that Java class from Python. 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.


Make sure to choose the right metaclass specifier. In Python 2 there is __metaclass__ class attribute, in Python 3 there is a new syntax class Stack(JavaClass, metaclass=MetaJavaClass).

For more info see PEP 3115.


Represents the Java class name, in the format ‘org/lang/Class’ (e.g. ‘java/util/Stack’), not ‘org.lang.Class’.


If not set, we assume the default constructor takes 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 with the method is automatically set from the declaration within the JavaClass itself.

The signature can be found with 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 try to access this method, it will choose the best method available according to the type of the arguments you’re using. Internally, we calculate a “match” score for each available signature, and take the best one. Without going into the details, the score calculation looks something 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 returns -1

Reflection functions#

jnius.autoclass(name, include_protected=True, include_private=True)#

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

By default, autoclass will include all fields and methods at all levels of the inheritance hierarchy. Use the include_protected and include_private parameters to limit visibility.

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

autoclass can also represent a nested Java class:

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


If a field and a method have the same name, the field will take precedence.


There are sometimes cases when a Java class contains a member that is a Python keyword (such as from, class, etc). You will need to use getattr() to access the member and then you will be able to call it:

from jnius import autoclass
func_from = getattr(autoclass('some.java.Class'), 'from')

There is also a special case for a SomeClass.class class literal which you will find either as a result of SomeClass.getClass() or in the __javaclass__ python attribute.


Currently SomeClass.getClass() returns a different Python object, therefore to safely compare whether something is the same class in Java use A.hashCode() == B.hashCode().

Java class implementation in Python#

class jnius.PythonJavaClass#

Base for creating a Java class from a Python class. This allows us to implement java interfaces completely in Python, and pass such a Python object back to Java.

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

You can only implement Java interfaces. You cannot sub-class a java object.

You must retain a reference to the Python object for the entire liftime that your object is in-use within java.

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

from jnius import PythonJavaClass, java_method

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

    def __init__(self, collection, index=0):
        super(PythonListIterator, 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’ (e.g. ‘java/util/Iterator’), not ‘org.lang.Class’.


Indicate which class loader to use, ‘system’ or ‘app’. The default is ‘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 look at the 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 indicate 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

The signature for methods of any android class can be easily seen by following these steps:

1. $ cd path/to/android/sdk/
2. $ cd platforms/android-xx/  # Replace xx with your android version
3. $ javap -s -classpath android.jar android.app.Activity  # Replace android.app.Activity with any android class whose methods' signature you want to see

Java Lambda implementation in Python using Lambdas and Function References#

It is possible to use Python lambdas or function references to implement Java functional interfaces <https://docs.oracle.com/javase/8/docs/api/java/util/function/package-summary.html#package.description>. A functional interface has one (non-default) method. When implementing a functional interface in Python, your lambda must have the correct number of parameters and return the correct data type. You must hold a reference to the Python lambda for as long as it will be used by Java.

For example, here we use a Python lambda to implement the Comparator <https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html> functional interface:

numbers = autoclass('java.util.ArrayList')()
Collections = autoclass('java.util.Collections')
revSort = lambda i, j: j - i
Collections.sort(numbers, revSort)

The lambda is wrapped in a PythonJavaClass, which implements the Java interface of the parameter in the called Java method.

Passing Variables: By Reference or By Value

When Python objects such as lists or bytearrays are passed to Java Functions, they are converted to Java arrays. Since Python does not share the same memory space as the JVM, a copy of the data needs to be made to pass the data.

Consider that the Java method might change values in the Java array. If the Java method had been called from another Java method, the other Java method would see the value changes because the parameters are passed by reference. The two methods share the same memory space. Only one copy of the array data exists.

In Pyjnius, Python calls to Java methods simulate pass by reference by copying the variable values from the JVM back to Python. This extra copying will have a performance impact for large data structures. To skip the extra copy and pass by value, use the named parameter pass_by_reference.

obj.method(param1, param2, param3, pass_by_reference=False)

Since Java does not have function named parameters like Python does, they are interpreted by Pyjnius and are not passed to the Java method.

In the above example, the pass_by_reference parameter will apply to all the parameters. For more control you can pass a list or tuple instead.

obj.method(param1, param2, param3, pass_by_reference=(False, True, False))

If the passed list or tuple is too short, the final value in the series is used for the remaining parameters.

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 use 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.

Pyjnius automatically calls this detach() function for you when a python thread exits. This is done by monkey-patching the default run() method of threading.Thread class.

So if you entirely override run() from your own subclass of Thread, you must call detach() yourself on any kind of termination.


import threading
import jnius

class MyThread(threading.Thread):

    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):