What is the native keyword in Java for?

问题:

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?

回答1:

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

回答2:

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

回答3:

Minimal example to make things clearer:

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 <jni.h>
#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:

It 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++?

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 *)&JVM_Clone},
};

JNIEXPORT void JNICALL
Java_java_lang_Object_registerNatives(JNIEnv *env, jclass cls)
{
    (*env)->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.

回答4:

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.

回答5:

As SLaks answered, the native keyword is for calling native code.

It also used by GWT for implementing javascript methods.

回答6:

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

回答7:

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

回答8:

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.

回答9:

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

回答10:

The native keyword is applied to a method to indicates that the method is implemented in native code using JNI (Java Native Interface). native is a modifier applicable only for methods and we can’t apply it anywhere else. The methods which are implemented in C, C++ are called as native methods or foreign methods.

The main objective of native keyword are:

• To improve performance of the system.
• To achieve mission level/memory level communication.
• To use already existing legacy non-java code.

回答11:

The native keyword is used to declare a method which is implemented in platform-dependent code such as C or C++. When a method is marked as native, it cannot have a body and must ends with a semicolon instead. The Java Native Interface (JNI)specification governs rules and guidelines for implementing native methods, such as data type conversion between Java and the native application.

The following example shows a class with a method declared as native:

public class NativeExample {
    public native void fastCopyFile(String sourceFile, String destFile);
}