2.让Android应用程序访问C库

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1>. JNI接口

<1. HardControl.java

<1. 作用：声明native方法
<2. 第一步：声明native方法

public static native int lcdCtrl(int which, int status);
public static native int ledOpen();
public static native void ledClose();

<3. 第二步：加载C库

System.loadLibrary(“hardcontrol”);

<2. hardcontrol.c

<1. 作用：实现相对应的C函数
<2. 相关变量

/* 定义一个本地JNI接口数组 */
<1. static const JNINativeMethod methods[] = {…}
/* 用于在被加载的第一时间： 1.获得运行环境 2.找到供应者 3.注册本地方法 */
<2. JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM *jvm, void *reserved)

2>. JNI字段描述符

变量类型
<1. jintArray等, 相关博文

3>. 开始狗血的写代码

. 编译源码：

<1. java源码：直接使用Android Studio
<2. C源码：最终要得到.so库
arm-linux-gcc -fPIC -shared hardcontrol.c -o libhardcontrol.so -I /usr/lib/jvm/java-1.7.0-openjdk-amd64/include/ -nostdlib /work/android-5.0.2/prebuilts/ndk/9/platforms/android-19/arch-arm/usr/lib/libc.so -I /work/android-5.0.2/prebuilts/ndk/9/platforms/android-19/arch-arm/usr/include/

. 将.so文件放入工程

<1. 第一步：创建armeabi目录：路径在\app\libs\下
<2. 第二步：修改build.gradle(module:app) 路径\app\src\build.gradle;
+ sourceSets {
main {
jniLibs.srcDirs = [‘libs’] //其实际作用是将libs设置为jni库文件的目录
}
}
[保证]：1.Android JNI层 HardControl.java : System.loadLibrary(“hardcontrol”); 库的名字一定要和HAL层的保持一致
2.Linux HAL层 hardcontrol.c : 编译出来的文件一定要保证，前缀为libxxx.so , 同时保证是放在创建好的armeabi目录中
同时里面的JNI_OnLoad() -> FindClass(env, “com/becauseican/hardlibrary/HardControl”); 在给出供应者目录的同时要保证附带了类的名字，区分大小写
同时JNINativeMethod methods[] 数组里面的注册函数，要保证与供应者所定义的函数名相同

4<. 错误汇总：

1.<运行报错：
dlopen failed: cannot locate symbol “__android_log_print” referenced by “libhardcontrol.so”…

解决方法>：
在编译选项后： + /work/android-5.0.2/prebuilts/ndk/9/platforms/android-19/arch-arm/usr/lib/liblog.so

2.<运行报错：
No implementation found for int com.becauseican.hardlibrary.HardControl.ledOpen();

解决方法>：
if ((*env)->RegisterNatives(env, cls, methods, (sizeof(methods) / sizeof(methods[0])) < 0))
语法问题!! : 请注意(sizeof(methods) / sizeof(methods[0])) < 0);

示例代码：

MainActivity.java – APP层

package com.becauseican.app_0001_leddemo;import android.app.Activity;import android.os.Bundle;import android.view.Menu;import android.view.MenuItem;import android.view.View;import android.widget.Button;import android.widget.CheckBox;import android.widget.Toast;import java.util.concurrent.Future;import com.becauseican.hardlibrary.*;public class MainActivity extends Activity {    private Button buttonLead = null;    private boolean boolLead = false;    private CheckBox checkBoxLed1 = null;    private CheckBox checkBoxLed2 = null;    private CheckBox checkBoxLed3 = null;    private CheckBox checkBoxLed4 = null;    /* 自己定义的类 */    class MyButtonListener implements View.OnClickListener {        @Override        public void onClick(View v) {            boolLead = !boolLead;            if (boolLead) {                buttonLead.setText("Close All");                checkBoxLed1.setChecked(true);                checkBoxLed2.setChecked(true);                checkBoxLed3.setChecked(true);                checkBoxLed4.setChecked(true);                /* 全部点亮 */                for (int i = 0; i < 4; ++i) {                    HardControl.ledCtrl(i, 1);                }            }else {                buttonLead.setText("Open All");                checkBoxLed1.setChecked(false);                checkBoxLed2.setChecked(false);                checkBoxLed3.setChecked(false);                checkBoxLed4.setChecked(false);                /* 全部熄灭 */                for (int i = 0; i < 4; ++i) {                    HardControl.ledCtrl(i, 0);                }            }        }    }    public void onCheckboxClicked(View view) {        // Is the view now checked?        boolean checked = ((CheckBox) view).isChecked();        // Check which checkbox was clicked        switch(view.getId()) {            case R.id.CheckBoxLed4:                if (checked) {                    Toast.makeText(getApplicationContext(), "CheckBoxLed4 on", Toast.LENGTH_SHORT).show();                    HardControl.ledCtrl(3, 1);                }                // Put some meat on the sandwich                else {                    Toast.makeText(getApplicationContext(), "CheckBoxLed4 off", Toast.LENGTH_SHORT).show();                    HardControl.ledCtrl(3, 0);                }                // Remove the meat                break;            case R.id.CheckBoxLed3:                if (checked) {                    Toast.makeText(getApplicationContext(), "CheckBoxLed3 on", Toast.LENGTH_SHORT).show();                    HardControl.ledCtrl(2, 1);                }                // Cheese me                else {                    Toast.makeText(getApplicationContext(), "CheckBoxLed3 off", Toast.LENGTH_SHORT).show();                    HardControl.ledCtrl(2, 0);                }                // I'm lactose intolerant                break;            case R.id.CheckBoxLed2:                if (checked) {                    Toast.makeText(getApplicationContext(), "CheckBoxLed2 on", Toast.LENGTH_SHORT).show();                    HardControl.ledCtrl(1, 1);                }                // Put some meat on the sandwich                else {                    Toast.makeText(getApplicationContext(), "CheckBoxLed2 off", Toast.LENGTH_SHORT).show();                    HardControl.ledCtrl(1, 0);                }                // Remove the meat                break;            case R.id.CheckBoxLed1:                if (checked) {                    Toast.makeText(getApplicationContext(), "CheckBoxLed1 on", Toast.LENGTH_SHORT).show();                    HardControl.ledCtrl(0, 0);                }                // Put some meat on the sandwich                else {                    Toast.makeText(getApplicationContext(), "CheckBoxLed1 off", Toast.LENGTH_SHORT).show();                    HardControl.ledCtrl(0, 0);                }                // Remove the meat                break;            // TODO: Veggie sandwich        }    }    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        buttonLead = (Button) findViewById(R.id.ButtonLead);        /* 实例化 HardControl 类对象*/        HardControl.ledOpen();        checkBoxLed1 = (CheckBox) findViewById(R.id.CheckBoxLed1);        checkBoxLed2 = (CheckBox) findViewById(R.id.CheckBoxLed2);        checkBoxLed3 = (CheckBox) findViewById(R.id.CheckBoxLed3);        checkBoxLed4 = (CheckBox) findViewById(R.id.CheckBoxLed4);/*      // 采用匿名类的方式        buttonLead.setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View v){                boolLead = !boolLead;                if (boolLead) {                    buttonLead.setText("Close All");                }else {                    buttonLead.setText("Open All");                }            }        });*/        //使用自定义类的方式        buttonLead.setOnClickListener(new MyButtonListener());    }    @Override    public boolean onCreateOptionsMenu(Menu menu) {        // Inflate the menu; this adds items to the action bar if it is present.        getMenuInflater().inflate(R.menu.menu_main, menu);        return true;    }    @Override    public boolean onOptionsItemSelected(MenuItem item) {        // Handle action bar item clicks here. The action bar will        // automatically handle clicks on the Home/Up button, so long        // as you specify a parent activity in AndroidManifest.xml.        int id = item.getItemId();        //noinspection SimplifiableIfStatement        if (id == R.id.action_settings) {            return true;        }        return super.onOptionsItemSelected(item);    }}

HardControl.java – APP层

package  com.becauseican.hardlibrary;public class HardControl {    public static native int ledCtrl(int which, int status);    public static native int ledOpen();    public static native void ledClose();    /* 静态构造块，所有的HardControl对象，只会执行一次 */    static {        try {            /* 加载C库 hardcontrol.so*/            System.loadLibrary("hardcontrol");        } catch (Exception e) {            e.printStackTrace();        }    }}

hardcontrol.c – JNI层

#include <jni.h>  /* /usr/lib/jvm/java-1.7.0-openjdk-amd64/include/ */#include <stdio.h>#include <stdlib.h>/* __android_log_print(ANDROID_LOG_DEBUG, "LEDDemo", "native ledopen.. : fd"); */#include <android/log.h>  /* liblog */#if 0typedef struct {    char *name;          /* Java里调用的函数名 */    char *signature;     /* JNI字段描述符, 用来表示Java里调用的函数的参数和返回值类型 */    void *fnPtr;         /* C语言实现的本地函数 */} JNINativeMethod;#endifjint ledOpen(JNIEnv *env, jobject cls){    //__android_log_print(ANDROID_LOG_DEBUG, "LEDDemo", "native ledopen..");     return 0;}void ledClose(JNIEnv *env, jobject cls) {    __android_log_print(ANDROID_LOG_DEBUG, "LEDDemo", "native ledClose.."); }jint ledCtrl(JNIEnv *env, jobject cls, jint which, jint status){    __android_log_print(ANDROID_LOG_DEBUG, "LEDDemo", "native ledCtrl.. which = %d status = %d : ", which, status);     return 0;}static const JNINativeMethod methods[] = {    {"ledOpen",  "()I", (void *)ledOpen},      /* ()I : 这个是JNI字段描述符 */    {"ledClose", "()V", (void *)ledClose},    {"ledCtrl",  "(II)I", (void *)ledCtrl},};/* System.loadLibrary */JNIEXPORT jint JNICALLJNI_OnLoad(JavaVM *jvm, void *reserved){    JNIEnv *env;    jclass cls;    /* 1.获得运行环境 */    if ((*jvm)->GetEnv(jvm, (void **)&env, JNI_VERSION_1_4)) {        return JNI_ERR; /* JNI version not supported */    }    /* 2.找到需要用到该文件的包：全路径 */    cls = (*env)->FindClass(env, "com/becauseican/hardlibrary/HardControl");    if (cls == NULL) {        return JNI_ERR;    }    /* 3.注册native 方法     */    if ((*env)->RegisterNatives(env, cls, methods, sizeof(methods) / sizeof(methods[0])) < 0)        return JNI_ERR;    /* 最终返回JNI接口版本 */    return JNI_VERSION_1_4;}

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