Java 外部函数接口：JNI, JNA, JNR https://nullwy.me/2018/01/java-ffi/
Java 本地方法调用实现调用 getpid 对比，JNI vs. JNA vs. JNR
JNI 示例编译与运行（Mac 环境）：
$ mkdir -p target/classes
$ javac src/main/java/com/test/GetPidJni.java -d "target/classes"
$ javah -cp "target/classes" com.test.GetPidJni
$ gcc -I $JAVA_HOME/include -I $JAVA_HOME/include/darwin -dynamiclib -o libgetpidjni.dylib com_test_GetPidJni.c
$ java -Djava.library.path=`pwd` -cp "target/classes" com.test.GetPidJni
JNA 示例编译与运行（包括 Interface Mapping 和 Direct Mapping 两种方式）：
$ mvn install
$ java -cp "target/java-ffi-1.0-SNAPSHOT-jar-with-dependencies.jar" com.test.GetPidJna
$ java -cp "target/java-ffi-1.0-SNAPSHOT-jar-with-dependencies.jar" com.test.GetPidJnaDirect
JNR 示例编译与运行：
$ mvn install
$ java -cp "target/java-ffi-1.0-SNAPSHOT-jar-with-dependencies.jar" com.test.GetPidJnr
JMH 性能测试代码：
$ mvn install
$ java -cp "target/java-ffi-1.0-SNAPSHOT-jar-with-dependencies.jar" com.test.BenchmarkFFI
JMH 性能测试结果：
# JMH version: 1.19
# VM version: JDK 1.8.0_144, VM 25.144-b01
Benchmark Mode Cnt Score Error Units
BenchmarkFFI.testGetPidJna thrpt 10 8225.209 ± 206.829 ops/ms
BenchmarkFFI.testGetPidJnaDirect thrpt 10 10257.505 ± 736.135 ops/ms
BenchmarkFFI.testGetPidJni thrpt 10 77852.899 ± 3167.101 ops/ms
BenchmarkFFI.testGetPidJnr thrpt 10 58261.657 ± 5187.550 ops/ms
即：JNI > JNR > JNA (Direct Mapping) > JNA (Interface Mapping)
参见资料：
2013-07 Charles Nutter: Java Native Runtime https://www.oracle.com/technetwork/java/jvmls2013nutter-2013526.pdf