Add lib to makefile for jni app

2 min read 07-10-2024

Adding External Libraries to Your JNI Application with Makefiles

Developing Java Native Interface (JNI) applications often involves utilizing external libraries to enhance functionality. When working with Makefiles, incorporating these libraries seamlessly is crucial for efficient build processes. This article will guide you through the process of integrating external libraries into your JNI project using Makefiles, ensuring a smooth and effective workflow.

Understanding the Challenge

The core challenge lies in ensuring that your Makefile correctly links the external library files with your JNI code. This involves defining the necessary compiler and linker flags to include the library headers and static/dynamic libraries during compilation and linking.

Scenario: Integrating an External Logging Library

Let's imagine you're developing a JNI application for Android and wish to integrate the popular Log4j library for logging purposes. Your current Makefile might look like this:

# Compiler and linker flags
CC = clang
CFLAGS = -g -Wall -I./include
LDFLAGS = -L./lib -llog4j

# Source files
SOURCES = main.c 

# JNI library name
JNI_LIB = libmyjni.so

# Build rules
all: $(JNI_LIB)

$(JNI_LIB): $(SOURCES)
	$(CC) $(CFLAGS) $(LDFLAGS) -shared -o $@ $(SOURCES)

Adding the External Library

To include the Log4j library, we need to modify our Makefile as follows:

Add the Library's Header Paths:
- Identify the directory containing Log4j's header files (e.g., /path/to/log4j/include).
- Update the CFLAGS to include this directory:
```
CFLAGS = -g -Wall -I./include -I/path/to/log4j/include 
```
Add the Library's Library Paths:
- Determine the directory containing Log4j's library files (e.g., /path/to/log4j/lib).
- Modify LDFLAGS to include this directory:
```
LDFLAGS = -L./lib -L/path/to/log4j/lib -llog4j
```
Specify the Library Name:
- The -llog4j flag informs the linker to link against the Log4j library. This flag might vary depending on the specific library (e.g., -lmylibrary for a library named libmylibrary.so).

Modified Makefile with Log4j Integration:

# Compiler and linker flags
CC = clang
CFLAGS = -g -Wall -I./include -I/path/to/log4j/include 
LDFLAGS = -L./lib -L/path/to/log4j/lib -llog4j

# Source files
SOURCES = main.c 

# JNI library name
JNI_LIB = libmyjni.so

# Build rules
all: $(JNI_LIB)

$(JNI_LIB): $(SOURCES)
	$(CC) $(CFLAGS) $(LDFLAGS) -shared -o $@ $(SOURCES)

Additional Considerations

Static vs. Dynamic Libraries: Make sure your Makefile handles both static and dynamic library linking correctly.
Library Dependencies: Be aware of any dependencies your external library might have on other libraries, and include those as well.
Cross-Platform Compatibility: Ensure that your Makefile works across different operating systems and architectures.
Documentation: Always consult the documentation for your external library to understand its specific requirements and linking instructions.

Conclusion

Integrating external libraries into your JNI projects using Makefiles is a common practice that significantly enhances functionality. By understanding the linking process and adapting your Makefile appropriately, you can ensure a smooth and efficient development experience. Remember to follow best practices and consult documentation for optimal results.