Android NDK JNI 入门笔记目录
Java & JNI 基本数据类型 Java 中的基本数据类型包括 boolean，byte，char，short，int，long，float，double 这几种。
而用 C/C++ 编写 native 代码时，是不能直接使用 Java 的数据类型的。
所以 JNI 提供了 jboolean、jbyte、jchar、jshort、jint、jlong、jfloat、jdouble。
Java 基本数据类型与 JNI 数据类型的映射关系

Java Type	Native Type	Description
boolean	jboolean	unsigned 8 bits
byte	jbyte	signed 8 bits
char	jchar	unsigned 16 bits
short	jshort	signed 16 bits
int	jint	signed 32 bits
long	jlong	signed 64 bits
float	jfloat	32 bits
double	jdouble	64 bits
void	void	N/A

基本数据类型传递-练习工程
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Java -> Native

点击每个按钮，将 Java 的基本数据类型传递到 Native，Native 代码（C/C++）中将数据打印到日志。

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JavaToNativeActivity.java

public class JavaToNativeActivity extends AppCompatActivity {@Override protected void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_java_to_native); setTitle("Java -> Native"); }public void booleanTest(View view) { NativeUtil.javaBooleanToNative(true); NativeUtil.javaBooleanToNative(false); }public void byteTest(View view) { NativeUtil.javaByteToNative(Byte.MIN_VALUE); NativeUtil.javaByteToNative(Byte.MAX_VALUE); NativeUtil.javaByteToNative(Byte.parseByte("123")); }public void charTest(View view) { NativeUtil.javaCharToNative(Character.MIN_VALUE); NativeUtil.javaCharToNative(Character.MAX_VALUE); NativeUtil.javaCharToNative('A'); NativeUtil.javaCharToNative('B'); NativeUtil.javaCharToNative('C'); }public void shortTest(View view) { NativeUtil.javaShortToNative(Short.MIN_VALUE); NativeUtil.javaShortToNative(Short.MAX_VALUE); NativeUtil.javaShortToNative(Short.parseShort("999")); }public void intTest(View view) { NativeUtil.javaIntToNative(Integer.MIN_VALUE); NativeUtil.javaIntToNative(Integer.MAX_VALUE); NativeUtil.javaIntToNative(Integer.parseInt("999999")); }public void longTest(View view) { NativeUtil.javaLongToNative(Long.MIN_VALUE); NativeUtil.javaLongToNative(Long.MAX_VALUE); NativeUtil.javaLongToNative(Long.parseLong("999999999")); }public void floatTest(View view) { NativeUtil.javaFloatToNative(Float.MIN_VALUE); NativeUtil.javaFloatToNative(Float.MAX_VALUE); NativeUtil.javaFloatToNative(999999.8F); }public void doubleTest(View view) { NativeUtil.javaDoubleToNative(Double.MIN_NORMAL); NativeUtil.javaDoubleToNative(Double.MAX_VALUE); NativeUtil.javaDoubleToNative(999999999.8); }}

NativeUtil.java

public class NativeUtil {static { System.loadLibrary("native-lib"); }public native static void javaBooleanToNative(boolean value); public native static void javaByteToNative(byte value); public native static void javaCharToNative(char value); public native static void javaShortToNative(short value); public native static void javaIntToNative(int value); public native static void javaLongToNative(long value); public native static void javaFloatToNative(float value); public native static void javaDoubleToNative(double value); }

native-lib.cpp

#include #include #include #include #define LOG_TAG"C_TAG" #define LOGD(...)__android_log_print(ANDROID_LOG_DEBUG, LOG_TAG, __VA_ARGS__) //LOGD("hello.length=%d",helloLen); extern "C" JNIEXPORT void JNICALL Java_com_ihubin_ndkjni_NativeUtil_javaBooleanToNative(JNIEnv *env, jclass clazz, jboolean value) { LOGD("Java Boolean: %d", value); }extern "C" JNIEXPORT void JNICALL Java_com_ihubin_ndkjni_NativeUtil_javaByteToNative(JNIEnv *env, jclass clazz, jbyte value) { LOGD("Java Byte: %d", value); }extern "C" JNIEXPORT void JNICALL Java_com_ihubin_ndkjni_NativeUtil_javaCharToNative(JNIEnv *env, jclass clazz, jchar value) { LOGD("Java Char: %d", value); }extern "C" JNIEXPORT void JNICALL Java_com_ihubin_ndkjni_NativeUtil_javaShortToNative(JNIEnv *env, jclass clazz, jshort value) { LOGD("Java Short: %d", value); }extern "C" JNIEXPORT void JNICALL Java_com_ihubin_ndkjni_NativeUtil_javaIntToNative(JNIEnv *env, jclass clazz, jint value) { LOGD("Java Int: %d", value); }extern "C" JNIEXPORT void JNICALL Java_com_ihubin_ndkjni_NativeUtil_javaLongToNative(JNIEnv *env, jclass clazz, jlong value) { LOGD("Java Long: %lld", value); }extern "C" JNIEXPORT void JNICALL Java_com_ihubin_ndkjni_NativeUtil_javaFloatToNative(JNIEnv *env, jclass clazz, jfloat value) { LOGD("Java Float: %f", value); }extern "C" JNIEXPORT void JNICALL Java_com_ihubin_ndkjni_NativeUtil_javaDoubleToNative(JNIEnv *env, jclass clazz, jdouble value) { LOGD("Java Double: %lf", value); }

点击每个按钮，在 Logcat 中查看输出日志

D/C_TAG: Java Boolean: 1 D/C_TAG: Java Boolean: 0 D/C_TAG: Java Byte: -128 D/C_TAG: Java Byte: 127 Java Byte: 123 D/C_TAG: Java Char: 0 Java Char: 65535 Java Char: 65 Java Char: 66 Java Char: 67 D/C_TAG: Java Short: -32768 D/C_TAG: Java Short: 32767 Java Short: 999 D/C_TAG: Java Int: -2147483648 Java Int: 2147483647 Java Int: 999999 D/C_TAG: Java Long: -9223372036854775808 Java Long: 9223372036854775807 Java Long: 999999999 D/C_TAG: Java Float: 0.000000 Java Float: 340282346638528859811704183484516925440.000000 Java Float: 999999.812500 D/C_TAG: Java Double: 0.000000 Java Double: 179769313486231570814527423731704356798070567525844996598917476803157260780028538760589558632766878171540458953514382464234321326889464182768467546703537516986049910576551282076245490090389328944075868508455133942304583236903222948165808559332123348274797826204144723168738177180919299881250404026184124858368.000000 Java Double: 999999999.800000

Java <- Navive

点击每个按钮，将从 Native 代码（C/C++）中获取 Java 的基本数据类型。

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NativeToJavaActivity.java

public class NativeToJavaActivity extends AppCompatActivity {private TextView booleanText; private TextView byteText; private TextView charText; private TextView shortText; private TextView intText; private TextView longText; private TextView floatText; private TextView doubleText; @Override protected void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_native_to_java); setTitle("Native -> Java"); booleanText = findViewById(R.id.boolean_text); byteText = findViewById(R.id.byte_text); charText = findViewById(R.id.char_text); shortText = findViewById(R.id.short_text); intText = findViewById(R.id.int_text); longText = findViewById(R.id.long_text); floatText = findViewById(R.id.float_text); doubleText = findViewById(R.id.double_text); }public void booleanTest(View view) { boolean value = https://www.it610.com/article/NativeUtil.javaBooleanFromNative(); booleanText.setText("value = "https://www.it610.com/article/+ value); }public void byteTest(View view) { byte value = NativeUtil.javaByteFromNative(); byteText.setText("value = "https://www.it610.com/article/+ value); }public void charTest(View view) { char value = NativeUtil.javaCharFromNative(); charText.setText("value = "https://www.it610.com/article/+ value); }public void shortTest(View view) { short value = NativeUtil.javaShortFromNative(); shortText.setText("value = "https://www.it610.com/article/+ value); }public void intTest(View view) { int value = NativeUtil.javaIntFromNative(); intText.setText("value = "https://www.it610.com/article/+ value); }public void longTest(View view) { long value = NativeUtil.javaLongFromNative(); longText.setText("value = "https://www.it610.com/article/+ value); }public void floatTest(View view) { float value = NativeUtil.javaFloatFromNative(); floatText.setText("value = "https://www.it610.com/article/+ value); }public void doubleTest(View view) { double value = NativeUtil.javaDoubleFromNative(); doubleText.setText("value = "https://www.it610.com/article/+ value); }}

NativeUtil.java

public class NativeUtil {static { System.loadLibrary("native-lib"); }public native static boolean javaBooleanFromNative(); public native static byte javaByteFromNative(); public native static char javaCharFromNative(); public native static short javaShortFromNative(); public native static int javaIntFromNative(); public native static long javaLongFromNative(); public native static float javaFloatFromNative(); public native static double javaDoubleFromNative(); }

native-lib.cpp

#include #include #include #include #define LOG_TAG"C_TAG" #define LOGD(...)__android_log_print(ANDROID_LOG_DEBUG, LOG_TAG, __VA_ARGS__) //LOGD("hello.length=%d",helloLen); extern "C" JNIEXPORT jboolean JNICALL Java_com_ihubin_ndkjni_NativeUtil_javaBooleanFromNative(JNIEnv *env, jclass clazz) { return JNI_TRUE; }extern "C" JNIEXPORT jbyte JNICALL Java_com_ihubin_ndkjni_NativeUtil_javaByteFromNative(JNIEnv *env, jclass clazz) { return 123; }extern "C" JNIEXPORT jchar JNICALL Java_com_ihubin_ndkjni_NativeUtil_javaCharFromNative(JNIEnv *env, jclass clazz) { return 'A'; }extern "C" JNIEXPORT jshort JNICALL Java_com_ihubin_ndkjni_NativeUtil_javaShortFromNative(JNIEnv *env, jclass clazz) { return 999; }extern "C" JNIEXPORT jint JNICALL Java_com_ihubin_ndkjni_NativeUtil_javaIntFromNative(JNIEnv *env, jclass clazz) { return 999999; }extern "C" JNIEXPORT jlong JNICALL Java_com_ihubin_ndkjni_NativeUtil_javaLongFromNative(JNIEnv *env, jclass clazz) { return 999999999; }extern "C" JNIEXPORT jfloat JNICALL Java_com_ihubin_ndkjni_NativeUtil_javaFloatFromNative(JNIEnv *env, jclass clazz) { return 999999.8; }extern "C" JNIEXPORT jdouble JNICALL Java_com_ihubin_ndkjni_NativeUtil_javaDoubleFromNative(JNIEnv *env, jclass clazz) { return 999999999.8; }

点击每个按钮，进行测试

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Java <-> Native

点击每个按钮，将和 Native(C/C++) 传递、获取 Java 的基本数据类型。

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JavaNativeActivity.java

public class JavaNativeActivity extends AppCompatActivity {private TextView charConcatText; private TextView sumText; private TextView twoExpText; private TextView calcMoneyText; @Override protected void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_java_native); setTitle("Java <-> Native"); charConcatText = findViewById(R.id.char_concat_text); sumText = findViewById(R.id.sum_text); twoExpText = findViewById(R.id.two_exp_text); calcMoneyText = findViewById(R.id.calc_money_text); }public void charConcatTest(View view) { String value = https://www.it610.com/article/NativeUtil.charConcatTest('A', 'B', 'C'); charConcatText.setText("A+B+C=" + value); }public void sumTest(View view) { int numOne = 123; int numTwo = 456; int value = https://www.it610.com/article/NativeUtil.sumText(numOne, numTwo); sumText.setText(numOne +"+" + numTwo + "=" + value); }public void twoExpTest(View view) { int value = https://www.it610.com/article/NativeUtil.twoExpTest(10); twoExpText.setText("2^10=" + value); }public void calcMoneyTest(View view) { double apple = 12.4; double banana = 99.8; double orange = 101.1; String value = https://www.it610.com/article/NativeUtil.calcMoneyTest(apple, banana, orange); calcMoneyText.setText(value); }}

NativeUtil.java

public class NativeUtil {static { System.loadLibrary("native-lib"); }public native static String charConcatTest(char a, char b, char c); public native static int sumText(int i, int j); public native static int twoExpTest(int exp); public native static String calcMoneyTest(double v, double v1, double v2); }

native-lib.cpp

#include #include #include #include #define LOG_TAG"C_TAG" #define LOGD(...)__android_log_print(ANDROID_LOG_DEBUG, LOG_TAG, __VA_ARGS__) //LOGD("hello.length=%d",helloLen); extern "C" JNIEXPORT jstring JNICALL Java_com_ihubin_ndkjni_NativeUtil_charConcatTest(JNIEnv *env, jclass clazz, jchar a, jchar b, jchar c) { char charArray[4]; charArray[0] = a; charArray[1] = b; charArray[2] = c; charArray[3] = '\0'; return env->NewStringUTF(charArray); }extern "C" JNIEXPORT jint JNICALL Java_com_ihubin_ndkjni_NativeUtil_sumText(JNIEnv *env, jclass clazz, jint i, jint j) { return i + j; }extern "C" JNIEXPORT jint JNICALL Java_com_ihubin_ndkjni_NativeUtil_twoExpTest(JNIEnv *env, jclass clazz, jint exp) { return pow(2, exp); }extern "C" JNIEXPORT jstring JNICALL Java_com_ihubin_ndkjni_NativeUtil_calcMoneyTest(JNIEnv *env, jclass clazz, jdouble v, jdouble v1, jdouble v2) { double totalMoney = v + v1 + v2; char *resultStr = new char(); sprintf(resultStr, "总计：%f", totalMoney); return env->NewStringUTF(resultStr); }

点击每个按钮，进行测试

【android|Android NDK JNI 入门笔记-day02-基本数据类型数据传递】
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至此，我们已经学会了在 Android 项目中与 Native(C/C++) 进行数据交换。
代码：
NDKJNIday02
参考资料：
Oracle - JNI Types and Data Structures
JNI 打印 log 信息

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