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

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native is a keyword in java   which is used to make unimplemented structure method  like as abstract but it would be a platform dependent such as native code and execute from native stack not java stack

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Minimal runnable example  Main java  public class Main       public native int square int i       public static void main String   args            System loadLibrary  Main            System out println new Main   square 2              Main c   include  lt jni h gt   include  Main h   JNIEXPORT jint JNICALL Java Main square      JNIEnv  env  jobject obj  jint i      return i   i      Compile and run   sudo apt-get install build-essential openjdk-7-jdk export JAVA HOME   usr lib jvm java-7-openjdk-amd64  javac Main java javah -jni Main gcc -shared -fpic -o libMain so -I  JAVA HOME  include     -I  JAVA HOME  include linux Main c java -Djava library path   Main   Output   4   Tested on Ubuntu 14 04 AMD64  Also worked with Oracle JDK 1 8 0 45   Example on GitHub for you to play with   Underscores in Java package   file names must be escaped with  1 in the C function name as mentioned at  Invoking JNI functions in Android package name containing underscore   Interpretation  native allows you to    call a compiled dynamically loaded library  here written in C  with arbitrary assembly code from Java and get results back into Java   This could be used to    write faster code on a critical section with better CPU assembly instructions  not CPU portable  make direct system calls  not OS portable    with the tradeoff of lower portability   It is also possible for you to call Java from C  but you must first create a JVM in C  How to call Java functions from C     Analogous native extension APIs are also present in many other  VM languages  for the same reasons  e g  Python  Node js  Ruby   Android NDK  The concept is exact the same in this context  except that you have to use Android boilerplate to set it up   The official NDK repository contains  canonical  examples such as the hello-jni app    https   github com googlesamples android-ndk blob 4df5a2705e471a0818c6b2dbc26b8e315d89d307 hello-jni app src main java com example hellojni HelloJni java L39 https   github com googlesamples android-ndk blob 4df5a2705e471a0818c6b2dbc26b8e315d89d307 hello-jni app src main cpp hello-jni c L27   In you unzip an  apk with NDK on Android O  you can see the pre-compiled  so that corresponds to the native code under lib arm64-v8a libnative-lib so   TODO confirm  furthermore  file  data app com android appname-  oat arm64 base odex  says it is a shared library  which I think is the AOT precompiled  dex corresponding to the Java files in ART  see also  What are ODEX files in Android  So maybe the Java is actually also run via a native interface   Example in the OpenJDK 8  Let s find find where Object clone is defined in jdk8u60-b27   We will conclude that it is implemented with a native call   First we find   find   -name Object java   which leads us to jdk src share classes java lang Object java l212   protected native Object clone   throws CloneNotSupportedException    Now comes the hard part  finding where clone is amidst all the indirection  The query that helped me was   find   -iname object c   which would find either C or C   files that might implement Object s native methods  It leads us to jdk share native java lang Object c l47   static JNINativeMethod methods                     clone            Ljava lang Object       void    amp JVM Clone       JNIEXPORT void JNICALL Java java lang Object registerNatives JNIEnv  env  jclass cls          env - gt RegisterNatives env  cls                              methods  sizeof methods  sizeof methods 0         which leads us to the JVM Clone symbol   grep -R JVM Clone   which leads us to hotspot src share vm prims jvm cpp l580   JVM ENTRY jobject  JVM Clone JNIEnv  env  jobject handle       JVMWrapper  JVM Clone      After expanding a bunch of macros  we come to the conclusion that this is the definition point

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It marks a method  that it will be implemented in other languages  not in Java  It works together with JNI  Java Native Interface    Native methods were used in the past to write performance critical sections but with Java getting faster this is now less common  Native methods are currently needed when   You need to call a library from Java that is written in other language  You need to access system or hardware resources that are only reachable from the other language  typically C   Actually  many system functions that interact with real computer  disk and network IO  for instance  can only do this because they call native code    See Also Java Native Interface Specification

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The native keyword is applied to a method to indicate that the method is implemented in native code using JNI  Java Native Interface

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native is a keyword in java  it indicates platform dependent  native methods are acts as interface between Java JNI  and other programming languages

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Straight from the Java Language Specification      A method that is native is implemented in platform-dependent code  typically written in another programming language such as C  C    FORTRAN or assembly language  The body of a native method is given as a semicolon only  indicating that the implementation is omitted  instead of a block

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NATIVE is Non access modifier it can be applied only to METHOD  It indicates the PLATFORM-DEPENDENT implementation of method or code

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functions that implement native code are declared native      The Java Native Interface  JNI  is a programming framework that enables Java code running in a Java Virtual Machine  JVM  to call  and to be called by  native applications  programs specific to a hardware and operating system platform  and libraries written in other languages such as C  C   and assembly    http   en wikipedia org wiki Java Native Interface

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As SLaks answered  the native keyword is for calling native code   It also used by GWT for implementing javascript methods

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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

[java] What is the native keyword in Java for?

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