面试-JNI

Android NDK 从入门到精通（汇总篇）(https://blog.csdn.net/afei__/article/details/81290711)

1.JNI demo

1.1 写java

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public class TestJni {
    static {
        System.load("/Users/liuming/_work/java/demo/jni/libhello.so");
    }

    private native static String hello(String message);

    public static void main(String[] args) {
        String text = hello("ming");
        System.out.println("Test jni-->" + text);

        String text1 = hello(null);
        System.out.println("Test jni-->" + text1);
    }
}

1.2 生成头文件

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# 自动生成native方法的头文件
java -h . TestJni.java
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/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class TestJni */

#ifndef _Included_TestJni
#define _Included_TestJni
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     TestJni
 * Method:    hello
 * Signature: (Ljava/lang/String;)Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_TestJni_hello
  (JNIEnv *, jclass, jstring);

#ifdef __cplusplus
}
#endif
#endif

1.3 实现头文件中声明的方法

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#include "TestJni.h"
#include <string.h>
#include <stdlib.h>
#include "/System/Library/Frameworks/JavaVM.framework/Versions/Current/Headers/jni.h"

JNIEXPORT jstring JNICALL Java_TestJni_hello(JNIEnv * env, jclass cls, jstring str) {
    if (!str) {
        return (*env)->NewStringUTF(env, "");
    }

    const char* s = (*env)->GetStringUTFChars(env, str, 0);
    const char* head_str = "nani:";
    char* dest = malloc(strlen(head_str) + strlen(s) + 1);
    if (dest == NULL) {
        fprintf(stderr, "%s\n", "error: malloc fail!");
        return (*env)->NewStringUTF(env, "");
    } 

    strcpy(dest, head_str);
    const char* res = strcat(dest, s);

    if (!dest) free(dest);
    if (!s) (*env)->ReleaseStringUTFChars(env, str, s);

    return (*env)->NewStringUTF(env, res);
}

1.4 编译

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# -I表示在这个目录中搜索用到的头文件, 这因为没写完整的jni.h的全路径，所以要加上
gcc -c -I "/System/Library/Frameworks/JavaVM.framework/Versions/Current/Headers" hello.c

# 把hello.o打包成libhello.so
gcc -shared -o libhello.so hello.o

# .java文件编译成.class
javac TestJni.java

1.5 运行

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java TestJni

2. 静态绑定&动态绑定

静态绑定

java_包名_类名_方法名
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JNIEXPORT jstring JNICALL Java_TestJni_hello(JNIEnv * env, jclass cls, jstring str);

动态绑定

原理：当java层调用System.load(“xxx.so”)时，会在so中搜索JNI_OnLoad, 然后被调用。
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jstring stringFromJNI(JNIEnv *env, jclass clazz) {
    return (*env)->NewStringUTF(env, "what are you fucking doing here?");
}

jint add(JNIEnv *env, jclass clazz, jint a, jint b) {
    return a + b;
}

// 方法签名
// |  java类型   |  类型描述符  |
// |  ----  | ----  |
// | int  | I |
// | long  | J |
// | byte  | B |
// | short  | S |
// | char  | C |
// | float  | F |
// | double  | D |
// | boolean  | Z |
// | void  | V |
// | 其他引用类型 | L+类全名+; |
// | 数组 | [ |
// | 方法 | (参数)返回值 |
JNIEXPORT jint JNICALL
RegisterNatives(JNIEnv *env) {
    jclass clazz = (*env)->FindClass(env, "TestJni");
    if (clazz == NULL) {
        printf("con't find class: TestJni");
        return JNI_ERR;
    }
    JNINativeMethod methods_TestJni[] = {
            {"stringFromJNI", "()Ljava/lang/String;", (void *) stringFromJNI},
            {"add",           "(II)I",                (void *) add}
    };
    int len = sizeof(methods_TestJni) / sizeof(methods_TestJni[0]);
    return (*env)->RegisterNatives(env, clazz, methods_TestJni, len);
}

JNIEXPORT jint JNICALL
JNI_OnLoad(JavaVM *vm, void *reserved) {
    JNIEnv *env = NULL;
    if ((*vm)->GetEnv(vm, (void **) &env, JNI_VERSION_1_6) != JNI_OK) {
        return JNI_ERR;
    }
    jint result = RegisterNatives(env);
    printf("RegisterNatives result: %d\n", result);
    return JNI_VERSION_1_6;
}

3.Android framework里，主要的jni的点