Detailed guide to classes and methods for calling JAR packages in Python

1. Prerequisites

Calling Java's JAR package in Python requires the following conditions:

Java environment: Make sure the system has Java Runtime Environment (JRE) or Java Development Kit (JDK) installed
- Recommended JDK 8 or higher
- Availablejava -versionCommand Verification
Python environment: It is recommended to use Python 3.6+
- Availablepython --versionCommand Verification
JAR package: The Java class that needs to be called must be packaged as a JAR file
- Ensure that classes and methods in JAR packages are accessible (public)

2. Main implementation methods

Method 1: Use JPype (recommended)

JPype is a Python library that allows Python code to call Java classes.

Installation dependencies

pip install JPype1

Sample code

import jpype

# Start JVM((), "-ea", "-=your_jar_file.jar")

# Import Java classesYourJavaClass = ("")

# Create an instanceinstance = YourJavaClass()

# Call methodresult = (param1, param2)

# Close the JVM (must be called at the end of the program)()

Method 2: Use PyJNIus

PyJNIus is another library for Python to interact with Java.

Installation dependencies

pip install pyjnius

Sample code

from jnius import autoclass

# Set the classpathimport os
['CLASSPATH'] = 'your_jar_file.jar'

# Load Java classesYourJavaClass = autoclass('')

# Create an instance and call a methodinstance = YourJavaClass()
result = (param1, param2)

Method 3: Use subprocess to call the command line

Suitable for simple command line call scenarios.

Sample code

import subprocess

# Call Java programresult = (['java', '-jar', 'your_jar_file.jar', 'arg1', 'arg2'], 
                        capture_output=True, text=True)

print()

3. Detailed steps (taking JPype as an example)

Prepare JAR package
- Make sure the JAR package contains the classes you need to call
- Understand the complete classpath (e.g.）
Setting up a Python environment

pip install JPype1

Write call code

import jpype

# Start the JVM and specify the classpathjvm_path = ()
classpath = "your_jar_file.jar"
(jvm_path, "-ea", f"-={classpath}")

# Load Java classestry:
    # Static method call example    System = ("")
    ("Hello from Java!")
    
    # Example method call    ArrayList = ("")
    list = ArrayList()
    ("item1")
    print(())
    
    # Call a class in a custom JAR    YourClass = ("")
    instance = YourClass()
    result = ("param1", 123)
    print(result)

finally:
    # Close JVM    ()

Handle data type conversion
- Automatic conversion of Python type to Java type:
  - int →
  - str →
  - list →
- Used when manual conversion is required

4. Things to note

JVM life cycle：
- The JVM can only be started once and must be closed at the end of the program.
- Avoid starting/shutting JVM multiple times in the same program
Memory management：
- Java objects will not be automatically garbage collected, and large-scale creation of objects may cause memory problems
- Consider manually releasing Java objects that are no longer needed
Thread safety：
- JPype is not thread-safe, ensures access to JVM from the same thread
Performance considerations：
- JVM startup has overhead, and frequent calls to simple methods may not be cost-effective
- For high-performance requirements, consider batch processing of data
Exception handling：

try:
    # Java code callsexcept  as e:
    print("Java exception:", ())
except Exception as e:
    print("Python exception:", str(e))

5. Frequently Asked Questions and Solutions

JVM or JVM path error not found
- Solution: explicitly specify the JVM path

('/path/to/jre/lib/server/', ...)

Class Not Found (ClassNotFoundException)
- Check if the classpath is correct
- Make sure the JAR package path is includedmiddle
- Check whether the package name and class name are correct
Method signature mismatch
- Java method overloading may cause call errors
- To specify the parameter type explicitly:

# For method overloadingresult = ((1), ("text"))

JVM is closed or not started
- Make sure the JVM is started before calling the Java code
- Check if it was called unexpectedlyshutdownJVM()
Performance issues
- Reduce data transfer between Python and Java
- Consider using batch operations instead of frequent small operations
32-bit/64-bit mismatch
- Make sure the number of bits of Python, Java, and JPype is consistent (both 32 or 64 bits)

6. Advanced usage

Callback mechanism: Call Python code in Java
- Python classes that implement Java interface
- useCreate a proxy for Java interface
Multi-threaded environment
- useattachThreadToJVM()anddetachThreadFromJVM()
- Pay attention to synchronization issues
Using Maven Dependencies
- Package all dependencies into one uber JAR
- Or add all JAR files to the classpath:

(classpath=["", "", ...])

Debugging Tips

Enable JVM debugging options:

(..., "-agentlib:jdwp=transport=dt_socket,server=y,suspend=y,address=5005")

- Then use the remote debugger to connect

Through the above methods, Java classes and methods in the JAR package can be flexibly called in Python to realize the interoperability between Python and Java. Choose the appropriate method according to specific needs, and pay attention to resource management and performance optimization.

This is the article about the detailed guide to classes and methods of Python calling JAR packages. For more related contents of Python calling JAR packages, please search for my previous articles or continue browsing the related articles below. I hope everyone will support me in the future!