How to Call Java Using Python with Jpype and Pyjnius?

In most of the current methods in computation, it is common to find a need to interface between two or more programming languages, particularly to take advantage of the optimized functionality that a language provides more so in handling specific problems. For instance, Java has been exceptionally strong in object-oriented capabilities, while Python is popular due to no-compromise ease of use and the speed of the code created. The combination of these two languages is often very useful, and Jpype and Pyjnius are two extremely valuable instruments to achieve it.

In this article, you'll learn how to call Java from Python with Jpype and Pyjnius: their installation and usage.

Jpype: A Seamless Bridge Between Python and Java

Jpype is a Python module that allows you to call Java functions from Python programs. It works by starting a Java Virtual Machine (JVM) within the Python process, enabling direct access to Java libraries.

Key Features of Jpype

• Direct Access to Java Libraries: You can access all Java libraries, including third-party libraries.
• Seamless Integration: Jpype uses Python's native syntax to interact with Java, reducing the learning curve.
• Automatic Type Conversion: Jpype automatically handles basic data type conversions (e.g., int, float, string).

Installation

To get started, you first need to install Jpype. You can easily do this using pip:

Basic Usage

Once installed, Jpype makes calling Java code simple and intuitive. Below is an example of how to invoke a Java class from Python:

File: sample.py

Output:

 List contains: [Python, Java] 

Explanation:

In this example, a Java `ArrayList` is created and manipulated in Python. Jpype makes this possible by allowing Python code to directly instantiate and use Java objects.

Pros of Jpype

1. Supports Complex Libraries: It's suitable for working with complex Java libraries or applications.
2. High Performance: Because it embeds the Java Virtual Machine (JVM) in Python, it offers good performance for large projects.

Cons of Jpype

1. JVM Overhead: Starting the Java Virtual Machine (JVM) inside Python can be resource intensive.
2. Complex Setup: For larger projects, configuring Jpype and handling JVM-related issues can be tricky.
3. Limited Multithreading: It has limitations when handling multithreaded Java applications within Python.
4. Java Dependency: It requires a properly configured Java environment, which can be an extra hurdle for some users.

Pyjnius: A Lightweight Solution for Java-Python Interoperability

Pyjnius is another Python library that allows calling Java classes from Python, focusing on ease of use and simplicity. Unlike Jpype, which starts a JVM inside Python, Pyjnius works by using JNI (Java Native Interface) to directly interface with Java.

Key Features of Pyjnius

• Lightweight: Pyjnius is more lightweight compared to Jpype, making it ideal for simpler projects.
• Direct Class Access: With `autoclass`, you can directly access and instantiate Java classes in Python.
• JNI-Based: Pyjnius uses JNI, making it efficient for cross-language calls but more challenging when dealing with large-scale Java libraries.

Installation

Like Jpype, Pyjnius can also be installed using pip:

Basic Usage

Here's a simple example demonstrating how to call Java classes using Pyjnius:

File: sample.py

Output:

 HELLO FROM JAVATPOINT! 

Explanation:

• This code snippet demonstrates the use of `autoclass`, which allows you to import and use any Java class in Python. Here, we're creating a Java `String` object and calling its `toUpperCase` method.

Pros of Pyjnius

1. No JVM Management: Pyjnius does not require explicitly managing the JVM; it handles it internally.
2. Good for Android: It is particularly useful for Android development, where Java methods need to be accessed from Python.

Cons of Pyjnius

1. Less Performance for Complex Apps: Pyjnius may struggle with performance in large, complex Java applications.
2. Manual Type Handling: It doesn't automatically handle Python-Java type conversion, requiring more manual work.
3. Limited Java Features: Pyjnius is more suited for simple use cases and may not fully support advanced Java libraries.
4. JNI Complexity: Being JNI-based, it can require more effort to troubleshoot if issues arise with Java classes or JNI compatibility.

Common Use Cases for Java-Python Interfacing

1. Machine Learning: Use Python for rapid development with TensorFlow or PyTorch, while Java handles backend logic and data processing.
2. Enterprise Applications: Java handles large-scale backend operations, while Python manages data analytics and automation.
3. Android Development: With Pyjnius, Python can call Java methods in Android development, enabling cross-platform mobile applications.

Conclusion

This way, with Jpype and Pyjnius you can learn from both and have a very friendly Python and Java integration. They allow strong cross-language interaction whether development of complex machine learning systems or development of new Android application. Choose the right tool based on your project's complexity and performance needs, and enjoy the best of both Python and Java.