如何在 Linux 上为 JNI 应用程序编译动态库？
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How to compile dynamic library for a JNI application on linux?
提问by dierre
I'm using Ubuntu 10.10
我正在使用Ubuntu 10.10
So that's what I did.
所以这就是我所做的。
Hello.java:
你好.java：
class Hello {
public native void sayHello();
static { System.loadLibrary("hellolib"); }
public static void main(String[] args){
Hello h = new Hello();
h.sayHello();
}
}
Then I ran the follwing commands:
然后我运行了以下命令：
dierre@cox:~/Scrivania/provajni$ javac Hello.java
dierre@cox:~/Scrivania/provajni$ javah -jni Hello
I've obtained Hello.classand Hello.h.
我已经获得Hello.class和Hello.h。
Hello.h:
你好.h:
/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class Hello */
#ifndef _Included_Hello
#define _Included_Hello
#ifdef __cplusplus
extern "C" {
#endif
/*
* Class: Hello
* Method: sayHello
* Signature: ()V
*/
JNIEXPORT void JNICALL Java_Hello_sayHello
(JNIEnv *, jobject);
#ifdef __cplusplus
}
#endif
#endif
Then I created Hello.cpp:
然后我创建了Hello.cpp：
#include <jni.h>
#include "Hello.h"
#include <iostream>
using namespace std;
JNIEXPORT void JNICALL Java_Hello_sayHello (JNIEnv *env, jobject obj) {
cout << "Hello World!" << endl;
return;
}
And now the part where I think I screwed up. I was inspiredby this guide (Compile the Dynamic or Shared Object Librarysection):
现在是我认为我搞砸的部分。我受到本指南的启发（编译动态或共享对象库部分）：
dierre@cox:~/Scrivania/provajni$ gcc -I"/usr/lib/jvm/java-6-sun/include" -I"/usr/lib/jvm/java-6-sun/include/linux" -o hellolib.so -shared -Wl,-soname,hello.so Hello.cpp -static -lc
that generates the file hellolib.so
生成文件 hellolib.so
But when I try to run it with java HelloI have this error:
但是当我尝试使用它运行它时，java Hello出现此错误：
Exception in thread "main" java.lang.UnsatisfiedLinkError: no hellolib in java.library.path
at java.lang.ClassLoader.loadLibrary(ClassLoader.java:1734)
at java.lang.Runtime.loadLibrary0(Runtime.java:823)
at java.lang.System.loadLibrary(System.java:1028)
at Hello.<clinit>(Hello.java:4)
Could not find the main class: Hello. Program will exit.
I even tried this:
我什至试过这个：
LD_LIBRARY_PATH=`pwd`
export LD_LIBRARY_PATH
with no results.
没有结果。
I know I'm doing something extremely stupid but I can't figure out what it is. The dynamic lib is generated with the -shared option, isn't it?
我知道我在做一些非常愚蠢的事情，但我无法弄清楚它是什么。动态库是使用 -shared 选项生成的，不是吗？
Update #1
更新 #1
I tried static { System.load("/home/dierre/Scrivania/provajni/hellolib.so"); }to see if that worked but now:
我试图static { System.load("/home/dierre/Scrivania/provajni/hellolib.so"); }看看这是否有效，但现在：
Exception in thread "main" java.lang.UnsatisfiedLinkError: /home/dierre/Scrivania/provajni/hello.so: /home/dierre/Scrivania/provajni/hello.so: undefined symbol: _ZSt4cout
at java.lang.ClassLoader$NativeLibrary.load(Native Method)
at java.lang.ClassLoader.loadLibrary0(ClassLoader.java:1803)
at java.lang.ClassLoader.loadLibrary(ClassLoader.java:1699)
at java.lang.Runtime.load0(Runtime.java:770)
at java.lang.System.load(System.java:1003)
at Hello.<clinit>(Hello.java:4)
Update #2Ok, to solve the Update #1problem I had to use g++insted of gcc, obviously. Still having trouble to use the loadmethod though. I can't seem to tell it the right path.
更新 #2好的，为了解决更新 #1问题，我必须使用g++insted gcc，显然。但是仍然无法使用该load方法。我似乎无法告诉它正确的道路。
采纳答案by qrtt1
Native library can be loaded by loadLibrary with a valid name. By example, libXXXX.so for linux family, your hellolib.so should rename to libhello.so. By the way, I develop java with jni, I will separate the implementation and native interface (.c or .cpp).
本机库可以由 loadLibrary 以有效名称加载。例如，对于 linux 系列，lib XXXX.so，您的 hellolib.so 应重命名为 libhello.so。顺便说一句，我用jni开发java，我会把实现和native interface（.c或.cpp）分开。
static {
System.loadLibrary("hello"); // will load libhello.so
}
The implementation header(HelloImpl.h):
实现头文件（HelloImpl.h）：
#ifndef _HELLO_IMPL_H
#define _HELLO_IMPL_H
#ifdef __cplusplus
extern "C" {
#endif
void sayHello ();
#ifdef __cplusplus
}
#endif
#endif
HelloImpl.cpp:
HelloImpl.cpp：
#include "HelloImpl.h"
#include <iostream>
using namespace std;
void sayHello () {
cout << "Hello World!" << endl;
return;
}
Hello.c (I prefer to compile jni in c):
Hello.c（我更喜欢用c编译jni）：
#include <jni.h>
#include "Hello.h"
#include "HelloImpl.h"
JNIEXPORT void JNICALL Java_Hello_sayHello (JNIEnv *env, jobject obj) {
sayHello();
return;
}
Finally, we can compile them in some steps:
最后，我们可以通过一些步骤来编译它们：
- compile obj (generate HelloImpl.o)
- 编译 obj（生成 HelloImpl.o）
g++ -c -I"/opt/java/include" -I"/opt/java/include/linux" HelloImpl.cpp
g++ -c -I"/opt/java/include" -I"/opt/java/include/linux" HelloImpl.cpp
- compile jni with .o
- 用 .o 编译 jni
g++ -I"/opt/java/include" -I"/opt/java/include/linux" -o libhello.so -shared -Wl,-soname,hello.so Hello.c HelloImpl.o -static -lc
g++ -I"/opt/java/include" -I"/opt/java/include/linux" -o libhello.so -shared -Wl,-soname,hello.so Hello.c HelloImpl.o -static -lc
in step 2, we use g++ to compile it. This is very important. yor can see How to mix C and C++
在第 2 步中，我们使用 g++ 来编译它。这是非常重要的。你可以看到如何混合 C 和 C++
After compilation, you can check the function naming with nm:
编译完成后，可以用nm检查函数命名：
$ nm libhello.so |grep say
00000708 T Java_Hello_sayHello
00000784 t _GLOBAL__I_sayHello
00000718 T sayHello
There is a Java_Hello_sayHello marked T. It should extactly equal to your native method name. If everything is ok. you can run it:
有一个标记为 T 的 Java_Hello_sayHello。它应该完全等于您的本地方法名称。如果一切正常。你可以运行它：
$ java -Djava.library.path=. Hello
Hello World!
回答by Gordon Thompson
This complains about the C++ symbols not being available. I seem to remember, when I use to do JNI stuff all of the time that there were problems linking in C++ libraries and we always stuck to plain old C
这抱怨 C++ 符号不可用。我似乎记得，当我一直在做 JNI 的事情时，在 C++ 库中存在链接问题，我们总是坚持使用普通的旧 C
If you change your code so that it's standard C (and rename the file):
如果您更改代码使其成为标准 C（并重命名文件）：
#include <jni.h>
#include "Hello.h"
#include <stdio.h>
JNIEXPORT void JNICALL Java_Hello_sayHello (JNIEnv *env, jobject obj) {
printf("Hello World");
return;
}
And compile it
并编译它
gcc -I/usr/lib/jvm/java-6-openjdk/include -o libhellolib.so -shared Hello.c
It works
有用
java -Djava.library.path=`pwd` Hello
Hello World
回答by Sandipan
Finally my code works. This is hello.java
最后我的代码有效。这是你好.java
public class hello {
public native void sayHello(int length) ;
public static void main (String args[]) {
String str = "I am a good boy" ;
hello h = new hello () ;
h.sayHello (str.length() ) ;
}
static {
System.loadLibrary ( "hello" ) ;
}
}
You should compile it as :
您应该将其编译为：
$ javac hello.java
To create .h file you should run this command:
要创建 .h 文件，您应该运行以下命令：
$ javah -jni hello
This is hello.h:
这是hello.h：
JNIEXPORT void JNICALL Java_hello_sayHello
(JNIEnv *, jobject, jint);
Here is hello.c:
这是hello.c：
#include<stdio.h>
#include<jni.h>
#include "hello.h"
JNIEXPORT void JNICALL Java_hello_sayHello
(JNIEnv *env, jobject object, jint len) {
printf ( "\nLength is %d", len ); }
To compile this and to create a shared library we have to run this command :
要编译它并创建共享库，我们必须运行以下命令：
$ gcc -I/usr/lib/jvm/java-6-openjdk/include -o libhello.so -shared hello.c
Then finally run this one :
然后最后运行这个：
$ java -Djava.library.path=. hello
