Java could call C API through Java Native Interfaces, we could implement that in following steps:
There should be a diagram to illustrate this process:

JAVA program –> JNI class –> .so/.dll (implement c API generated from JNI class) –> C APIs we want to call

==========================================

  1. Build a Java class, declaring the native interfaces. These native interfaces are called by other classes who want to call the C APIs.

a example of JNI class:

package coresystem; 
class Adapter { 
    native String call(String in); 
    static {
        System.loadLibrary("coresystem"); 
    }
}

Note:
a. We must declare all the APIs which we want to call them in java into native methods;
b. Native methods are loaded with the System.loadLibrary method. The argument to System.loadLibrary is a library name chosen arbitrarily by the programmer. The system follows a standard, but platform-specific, approach to convert the library name to a native library name. For example, a Solaris system converts the name coresystem to libcoresystem.so, while a Win32 system converts the same coresystem name to coresystem.dll.

  1. Compiling:

creating class file:

javac coresystem/Adapter.java

creating head file for native library:

javah coresystem.Adapter

You could see a file named coresystem_Adapter.h was generated.

  1. implement the C APIs descripted in coresystem_Adapter.h and create a .so file.
    In this .so file, there should be the logic to call the C APIs that Java want to call.

    export JAVA_HOME=/usr/java/jdk1.7.0_45
    export PATH=$JAVA_HOME/bin:$PATH
    export LD_LIBRARY_PATH=$JAVA_HOME/jre/lib/amd64/server/:$LD_LIBRARY_PATH
    export C_INCLUDE_PATH=$JAVA_HOME/include:$JAVA_HOME/include/linux
    gcc -g coresystem.c -o libcoresystem.so -shared

  2. set environment for the new generated .so:

    export LD_LIBRARY_PATH=${LD_LIBRARY_PATH}:/opt/jc

  3. Run Java program, it works:

    package coresystem;
    public class Test {

    public static final void main(String[] args){
        Adapter adapter = new Adapter();
        String response = adapter.call(args[0]);
        System.out.println(response);
    }
    

    }

complie and run:

javac coresystem/Test.java
java coresystem.Test hello