MainActivity：

package com.itheima.monitor;import android.os.Bundle;import android.app.Activity;import android.view.Menu;import android.view.View;import android.widget.ProgressBar;public class MainActivity extends Activity {    private MyProgressBar pb;    static{        System.loadLibrary("hello");    }    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        pb = (MyProgressBar) findViewById(R.id.pb);//自定义进度条        pb.setMax(100);    }   public void start(View v){       new Thread(){           public void run() {               startMonitor();           };       }.start();   }   public void stop(View v){       stopMonitor();   }       public native void startMonitor();   public native void stopMonitor();      public void show(int pressure){       pb.setPressure(pressure);   }}

MyProgressBar。java

package com.itheima.monitor;import android.content.Context;import android.graphics.Canvas;import android.graphics.Color;import android.graphics.Paint;import android.util.AttributeSet;import android.view.View;/*界面：
     */public class MyProgressBar extends View {    private int pressure;    private int max;    public MyProgressBar(Context context, AttributeSet attrs, int defStyle) {        super(context, attrs, defStyle);    }    public MyProgressBar(Context context, AttributeSet attrs) {        super(context, attrs);    }    public MyProgressBar(Context context) {        super(context);    }    public void setPressure(int pressure) {        this.pressure = pressure;        //使当前组件MyProgressBar视图无效，如果当前视图是可见的，那么onDraw会调用，只能在主线程使用//        invalidate();  主线程调用这个方法        postInvalidate();//子线城用这个方法，这样进度条才会一直绘制进度条。            }    public void setMax(int max) {        this.max = max;    }        //在自定义组件绘制内容    @Override    protected void onDraw(Canvas canvas) {        //任何组件在显示在页面上的时候onDraw方法都会调用一次，        Paint paint = new Paint();//画笔        if(pressure < 50){            paint.setColor(Color.GREEN);        }        else if(pressure < 80){            paint.setColor(0xffff8800);        }        else {            paint.setColor(Color.RED);        }        canvas.drawRect(10, 10 + max - pressure, 30, 10 + max, paint);//画矩形,也就是画进度条。        canvas.drawText("dddd", 10, 10, paint);//写文字    }}

hello.c

#include 
     
      #include 
      
       #include 
       
        int getPressure(){    //……监测电压……运算得到压力值    return rand() % 101;}int monitor;JNIEXPORT void JNICALL Java_com_itheima_monitor_MainActivity_startMonitor  (JNIEnv * env, jobject obj){
   //obj是MainActivity，    monitor = 1;    while(monitor){        //获取锅炉压力        int pressure = getPressure();//C中通过反射调用java的方法        //jclass  (*FindClass)(JNIEnv*, const char*);        jclass clazz = (*env)->FindClass(env, "com/itheima/monitor/MainActivity");        //jmethodID  (*GetMethodID)(JNIEnv*, jclass, const char*, const char*);        jmethodID methodId = (*env)->GetMethodID(env, clazz, "show", "(I)V");//(I)V是方法的签名，返回值是void参数是int        //void  (*CallVoidMethod)(JNIEnv*, jobject, jmethodID, ...);        (*env)->CallVoidMethod(env, obj, methodId, pressure);//调用java的show方法        sleep(1);    }}JNIEXPORT void JNICALL Java_com_itheima_monitor_MainActivity_stopMonitor (JNIEnv * env, jobject obj){    monitor = 0;}//进入工程的bin\classes目录，执行javap -s Mainactivity的全包名类名，就可以查看MainActivity的所有方法的签名。