第四章之刻意练习
Practice 1
在Java侧定义两个成员变量，一个为静态成员变量，另一个为非静态成员变量。在Java侧初始化并打印这两个值，然后再native侧修改这两个值，返回到Java侧后再打印这两个值。定义这两个值一个为String类型，另一个为double类型好了。
** 下面是博主写的答案 **
Java侧代码：
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class Practice1 { static String string; double dd; private native void access_modify(); public static void main(String[] args) { Practice1 p = new Practice1(); Practice1.string = "Hello World!"; p.dd = 12.3; System.out.println("static string: " + Practice1.string); System.out.println("instance double: " + p.dd); p.access_modify(); System.out.println("modify static string: " + Practice1.string); System.out.println("modify instance double: " + p.dd); return; } static { System.loadLibrary("Practice1"); } } |
native侧代码：
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#include <jni.h> #include <stdio.h> JNIEXPORT void JNICALL native_access_modify(JNIEnv * env, jobject obj) { jfieldID instance_id, static_id; jclass clazz; jstring str; jdouble jdd; const char * c_str; // 获取实例对象object所在的类 clazz = (*env)->GetObjectClass(env, obj); if(clazz == NULL) { printf("GetObjectClass return error.\n"); return ; } // 获取类实例成员ID instance_id = (*env)->GetFieldID(env, clazz, "dd", "D"); if(instance_id == NULL) { printf("GetFiledID return error.\n"); return; } // 获取静态成员ID static_id = (*env)->GetStaticFieldID(env, clazz, "string", "Ljava/lang/String;"); if(static_id == NULL) { printf("GetStaticFieldID return error.\n"); return; } // 获取类实例成员的值 jdd = (*env)->GetDoubleField(env, obj, instance_id); printf("In native, jdd = %lf\n", jdd); // 设置类实例成员的值 (*env)->SetDoubleField(env, obj, instance_id, 32.1); // 获取类静态成员的值 str = (*env)->GetStaticObjectField(env, clazz, static_id); c_str = (*env)->GetStringUTFChars(env, str, NULL); if(c_str == NULL) { printf("GetStringUTFChars return error.\n"); return; } printf("In native, c_str = %s\n", c_str); (*env)->ReleaseStringUTFChars(env, str, c_str); // 设置类静态成员的值 str = (*env)->NewStringUTF(env, "Hello Jimmy Chen!"); if(str == NULL) { printf("NewStringUTF return error.\n"); return; } (*env)->SetStaticObjectField(env, clazz, static_id, str); return; } static const JNINativeMethod gMethods[] = { {"access_modify", "()V", native_access_modify} }; static jclass myClass; static const char * ClassName = "Practice1"; JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM * vm, void * reversed) { JNIEnv * env = NULL; if((*vm)->GetEnv(vm, (void **)&env, JNI_VERSION_1_6) != JNI_OK) { printf("GetEnv return error.\n"); return -1; } myClass = (*env)->FindClass(env, ClassName); if(myClass == NULL) { printf("FindClass return error.\n"); return -1; } if((*env)->RegisterNatives(env, myClass, gMethods, sizeof(gMethods)/sizeof(gMethods[0])) < 0) { printf("RegisterNatives return error.\n"); return -1; } return JNI_VERSION_1_6; } |
Practice 2
在Java侧定义四则运算加减乘除，然后在native侧获取两个double输入，分别调用Java侧的四则运算进行求值。
** 下面是博主自己写的答案 **
Java侧代码：
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class Practice2 { private native void add_sub(); public native void mul_div(); private double add(double a, double b) { return a+b; } public double sub(double a, double b) { return a-b; } public static double mul(double a, double b) { return a*b; } private static double div(double a, double b) { return a/b; } public static void main(String args[]) { Practice2 p = new Practice2(); p.add_sub(); p.mul_div(); } static { System.loadLibrary("Practice2"); } } |
Native侧代码：
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#include <jni.h> #include <stdio.h> JNIEXPORT void JNICALL native_add_sub(JNIEnv * env, jobject obj) { jmethodID add_id, sub_id; jclass clazz; jdouble a, b; jdouble result; // 获取类 clazz = (*env)->GetObjectClass(env, obj); if(clazz == NULL) { printf("GetObjectClass return NULL.\nm"); return; } // add方法ID add_id = (*env)->GetMethodID(env, clazz, "add", "(DD)D"); if(add_id == NULL) { printf("get add_id return NULL.\n"); return; } // sub方法ID sub_id = (*env)->GetMethodID(env, clazz, "sub", "(DD)D"); if(clazz == NULL) { printf("get sub_id return NULL,\n"); return; } printf("Enter the first double number: "); scanf("%lf", &a); printf("Enter the second double number: "); scanf("%lf", &b); // 调用方法，传递a和b变量 result = (*env)->CallDoubleMethod(env, obj, add_id, a, b); printf("Call add method, %lf + %lf = %lf\n", a, b, result); result = (*env)->CallDoubleMethod(env, obj, sub_id, a, b); printf("Call sub method, %lf - %lf = %lf\n", a, b, result); return; } JNIEXPORT void JNICALL native_mul_div(JNIEnv * env, jobject obj) { jmethodID mul_id, div_id; jclass clazz; jdouble a, b; jdouble result; clazz = (*env)->GetObjectClass(env, obj); if(clazz == NULL) { printf("GetObjectClass return NULL.\n"); return; } // 获取静态方法ID mul_id = (*env)->GetStaticMethodID(env, clazz, "mul", "(DD)D"); if(mul_id == NULL) { printf("get mul_id return NULL.\n"); return; } // 获取静态方法ID div_id = (*env)->GetStaticMethodID(env, clazz, "div", "(DD)D"); if(div_id == NULL) { printf("get div_id return NULL.\n"); return; } printf("Enter the first double number: "); scanf("%lf", &a); printf("Enter the second double number: "); scanf("%lf", &b); result = (*env)->CallStaticDoubleMethod(env, clazz, mul_id, a, b); printf("Call mul method, %lf * %lf = %lf\n", a, b, result); result = (*env)->CallStaticDoubleMethod(env, clazz, div_id, a, b); printf("Call mul method, %lf / %lf = %lf\n", a, b, result); return; } const static JNINativeMethod gMethods[] = { {"add_sub", "()V", native_add_sub}, {"mul_div", "()V", native_mul_div} }; static jclass myClass; static const char * ClassName = "Practice2"; JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM * vm, void * reversed) { JNIEnv * env = NULL; if((*vm)->GetEnv(vm, (void **)&env, JNI_VERSION_1_6) != JNI_OK) { printf("GetEnv return error.\n"); return -1; } myClass = (*env)->FindClass(env, ClassName); if(myClass == NULL) { printf("FindClass return error.\n"); return -1; } if((*env)->RegisterNatives(env, myClass, gMethods, sizeof(gMethods)/sizeof(gMethods[0])) < 0) { printf("RegisterNatives return error.\n"); return -1; } return JNI_VERSION_1_6; } |
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