What is the native keyword in Java for?

Question

While playing this puzzle (It\'s a Java keyword trivia game), I came across the native keyword.

What is the native keyword in Java used for?

Answer 1

Java native method provides a mechanism for Java code to call OS native code, either due to functional or performance reasons.

Example:

• java.lang.Rutime (source code on github) contains the following native method definition

606  public native int availableProcessors();
617  public native long freeMemory();
630  public native long totalMemory();
641  public native long maxMemory();
664  public native void gc();

In the corresponding Runtime.class file in OpenJDK, located in JAVA_HOME/jmods/java.base.jmod/classes/java/lang/Runtime.class, contains these methods and tagged them with ACC_NATIVE (0x0100), and these methods do not contain the Code attribute, which means these method do not have any actual coding logic in the Runtime.class file:

• Method 13 availableProcessors: tagged as native and no Code attribute
• Method 14 freeMemory: tagged as native and no Code attribute
• Method 15 totalMemory: tagged as native and no Code attribute
• Method 16 maxMemory: tagged as native and no Code attribute
• Method 17 gc: tagged as native and no Code attribute

The in fact coding logic is in the corresponding Runtime.c file:

42  #include "java_lang_Runtime.h"
43
44  JNIEXPORT jlong JNICALL
45  Java_java_lang_Runtime_freeMemory(JNIEnv *env, jobject this)
46  {
47      return JVM_FreeMemory();
48  }
49
50  JNIEXPORT jlong JNICALL
51  Java_java_lang_Runtime_totalMemory(JNIEnv *env, jobject this)
52  {
53      return JVM_TotalMemory();
54  }
55
56  JNIEXPORT jlong JNICALL
57  Java_java_lang_Runtime_maxMemory(JNIEnv *env, jobject this)
58  {
59      return JVM_MaxMemory();
60  }
61
62  JNIEXPORT void JNICALL
63  Java_java_lang_Runtime_gc(JNIEnv *env, jobject this)
64  {
65      JVM_GC();
66  }
67  
68  JNIEXPORT jint JNICALL
69  Java_java_lang_Runtime_availableProcessors(JNIEnv *env, jobject this)
70  {
71      return JVM_ActiveProcessorCount();
72  }

And these C coding is compiled into the libjava.so (Linux) or libjava.dll (Windows) file, located at JAVA_HOME/jmods/java.base.jmod/lib/libjava.so:

Reference

• Java Native Methods Essentials