java JNI介紹
java JNI介紹
JNI是Java Native Interface的全稱。
oracle文檔中是這樣描述的
The JNI is a native programming interface. It allows Java code that runs inside a Java Virtual Machine (VM) to interoperate with applications and libraries written in other programming languages, such as C, C++, and assembly.
翻譯過來就是 JNI 是本機編程介面。 它允許在 Java 虛擬機 (VM) 內運行的 Java 程式碼與使用其他程式語言（例如 C、C++ 和彙編）編寫的應用程式和庫進行互操作。
說白了就是java程式碼和其他程式語言的程式碼相互調用。
今天就主要記錄下日常的一些使用方式
1、 Java調用C++程式碼
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首先創建我們的java類 JNIDemo.java
package com.wbo112.jni; import java.util.*; import java.util.concurrent.*; public class JNIDemo { public native String callHello(); public native boolean sendMsg(String str); //保存JNI返回的結果 private BlockingQueue<Entry> queue = new ArrayBlockingQueue<Entry>(10); //處理jni返回的任務 執行緒池 private ExecutorService executor = Executors.newSingleThreadExecutor(); public static void main(String[] args) throws InterruptedException { //載入so文件 libJNIDemo.so System.loadLibrary("JNIDemo"); //System.load(); 也可以用這種方式，參數是so文件的全路徑 JNIDemo jniDemo = new JNIDemo(); //保存需要提交的任務 Set<String> sets = new ConcurrentSkipListSet<>(); String str; for (int i = 0; i < 10; i++) { str = UUID.randomUUID().toString(); sets.add(str); System.out.println("commit task :" + str); //在這裡會調用jni 提交任務，jni中會新啟動一個執行緒，非同步去執行任務，執行完了會調用putEntry方法，添加到需要回調的任務列表中 jniDemo.sendStr(str); } Thread thread = new Thread(() -> { while (true) { try { Entry entry = jniDemo.queue.take(); sets.remove(entry.str); //在執行緒中中，進行回調通知，比如向調用方發送任務處理結果 jniDemo.executor.execute(() -> System.out.println(entry.str + "process finish")); //所有任務進行回調後，結束添加回調任務執行緒，同時關閉執行緒池 if (sets.isEmpty()) { jniDemo.executor.shutdown(); break; } } catch (InterruptedException e) { System.out.println("thread interrupt "); jniDemo.executor.shutdown(); break; } } }); thread.start(); //也可以通過這種方式，比如在其他地方在中斷回調任務的添加執行，關閉執行緒池 //thread.interrupt(); } public boolean sendStr(String str) { return sendMsg(str); } public void putEntry(Entry entry) { queue.add(entry); } private static class Entry { //表示一個任務 private String str; //表示任務處理結果 private String result; public String getStr() { return str; } public void setStr(String str) { this.str = str; } public String getResult() { return result; } public void setResult(String result) { this.result = result; } @Override public String toString() { return "Entry{" + "str='" + str + '\'' + ", result='" + result + '\'' + '}'; } } } -
編譯成class文件
javac com/wbo112/jni/JNIDemo.java -
生成頭文件
javah com.wbo112.jni.JNIDemo這時就會在當前目錄下生成com_wbo112_jni_JNIDemo.h。
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編寫對應的cpp文件 JNIDemo1.cpp
#include "com_wbo112_jni_JNIDemo.h" #include <iostream> #include <thread> #include <cstdlib> #include <ctime> #include <unistd.h> JavaVM *vm=NULL; unsigned seed; //在java載入so文件的時候，就會調用到這個JNI_OnLoad //oracle文檔這裡有介紹：//docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/invocation.html#JNJI_OnLoad jint JNI_OnLoad(JavaVM *jvm, void *reserved){ //這個表示java虛擬機,這個需要保存下來，因為JNIEnv只是當前執行緒有效，如果要在其他執行緒獲取JNIEnv,就需要通過這個jvm來獲取，後面有相應程式碼 vm=jvm; //這個是為了模擬後面的回調時間，業務中應該不關注 seed = time(0); srand(seed); return JNI_VERSION_1_8; } ////docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/invocation.html#JNI_OnUnload //so文件卸載的時候，會執行這個函數，在這裡面可以做一些清理工作 void JNI_OnUnload_L(JavaVM *vm, void *reserved){ vm=NULL; } void threadfunc(jobject jobj,jstring jstr) { JNIEnv* env = NULL; sleep(rand() % 10); ////docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/invocation.html#GetEnv //這裡有介紹，如果不是當前執行緒，是獲取不到env的，這時，返回值就是JNI_EDETACHED,這時就需要調用AttachCurrentThread，給當前執行緒綁定env jint status = vm->GetEnv((void **)&env, JNI_VERSION_1_8); if (status == JNI_EDETACHED) { ////docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/invocation.html#AttachCurrentThread if (vm->AttachCurrentThread((void **)&env, NULL)!= JNI_OK){ return; } } else if(status!= JNI_OK){ std::cout<<"getEnv err"<<std::endl; return; } //這個是將jstring轉成char* const char *str = env->GetStringUTFChars(jstr, 0); std::cout << "start process task " + (std::string)str << std::endl; //通過對象獲取對應的類 ////docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#GetObjectClass jclass jcls=env->GetObjectClass(jobj); //另一種方式獲取對應的類 ////docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#FindClass jclass jentrycls=env->FindClass("com/wbo112/jni/JNIDemo$Entry"); //獲取無參的構造方法 ////docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#GetMethodID jmethodID jinit=env->GetMethodID(jcls,"<init>", "()V"); //調用構造方法獲取對象 ////docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#NewObject jobject jentryobj=env->NewObject(jentrycls,jinit); //獲取entry類的str欄位 ////docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#GetFieldID jfieldID jfieldStr=env->GetFieldID(jentrycls,"str","Ljava/lang/String;"); //給str欄位設置值 ////docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#Set_type_Field_routines env->SetObjectField(jentryobj,jfieldStr,jstr); char* msg = "Hello World!"; jstring result = env->NewStringUTF(msg); jfieldID jfieldResult=env->GetFieldID(jentrycls,"result","Ljava/lang/String;"); env->SetObjectField(jentryobj,jfieldResult,result); //獲取方法 ////docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#GetMethodID jmethodID jmtd=env->GetMethodID(jcls,"putEntry", "(Lcom/wbo112/jni/JNIDemo$Entry;)V"); //調用方法 ////docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#Call_type_Method_routines env->CallVoidMethod(jobj,jmtd,jentryobj); std::cout<<" end process task " + (std::string)str<<std::endl; //釋放前面構造的char* env->ReleaseStringUTFChars( jstr, str); //釋放全局對象 ////docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#DeleteGlobalRef env->DeleteGlobalRef(jobj); env->DeleteGlobalRef(jstr); //當前執行緒和jvm進行分離 ////docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/invocation.html#DetachCurrentThread vm->DetachCurrentThread(); } JNIEXPORT jstring JNICALL Java_com_wbo112_jni_JNIDemo_callHello (JNIEnv *env, jobject jobj){ char* msg = "Hello World!"; jstring result = env->NewStringUTF(msg); // C style string to Java String return result; } JNIEXPORT jboolean JNICALL Java_com_wbo112_jni_JNIDemo_sendMsg (JNIEnv *env, jobject jobj, jstring jstr){ //jobject對象是不能跨執行緒傳遞的，需要先轉成全局引用 ////docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#NewGlobalRef jobject globalJobj=env->NewGlobalRef(jobj); jobject globalJstr=env->NewGlobalRef(jstr); //這裡使用的是std::thread ,所以編譯的時候需要加參數-std=c++11 std::thread t1(threadfunc,globalJobj,(jstring)globalJstr); //t1.join(); t1.detach(); return 1; }編譯cpp文件
g++ -shared -std=c++11 -I $JAVA_HOME/include/linux -I $JAVA_HOME/include -fPIC -o libJNIDemo.so JNIDemo1.cpp -
執行java程式
java -Djava.library.path=./ com.wbo112.jni.JNIDemo
2、C++程式碼調用java程式碼
這個完全就是Oracle官方的例子了
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首先創建個ctj.cpp文件
#include <jni.h> /* where everything is defined */ int main(){ JavaVM *jvm; /* denotes a Java VM */ JNIEnv *env; /* pointer to native method interface */ JavaVMInitArgs vm_args; /* JDK/JRE 6 VM initialization arguments */ JavaVMOption* options = new JavaVMOption[1]; options[0].optionString = "-Djava.class.path=./"; vm_args.version = JNI_VERSION_1_8; vm_args.nOptions = 1; vm_args.options = options; vm_args.ignoreUnrecognized = false; /* load and initialize a Java VM, return a JNI interface * * pointer in env */ JNI_CreateJavaVM(&jvm, (void**)&env, &vm_args); delete options; /* invoke the Main.test method using the JNI */ jclass cls = env->FindClass("Main"); jmethodID mid = env->GetStaticMethodID(cls, "test", "(I)V"); env->CallStaticVoidMethod(cls, mid, 100); /* We are done. */ jvm->DestroyJavaVM(); return 0; } -
再創建c++需要調用的java文件 Main.java
public class Main { public static void test(int a) { System.out.println(" Main test:" + a); } } -
編譯ctj.cpp文件
g++ -I $JAVA_HOME/include/linux -I $JAVA_HOME/include -L"$JAVA_HOME/jre/lib/amd64/server/" ctj.cpp -o ctj -ljvm -
執行生成的ctj文件
export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:$JAVA_HOME/jre/lib/amd64/server/ ./ctj
