jni调用的函数和native函数不在一个java文件内，调用出错

Question

问题描述如题。下来我贴代码：
这是含有native函数的java文件

 public class ProgressBarOperation {
    static {
        System.loadLibrary("Pressure");
    }

    public native void startMonitor();

    public native void stopMonitor();
}

native函数所在的java文件不能继承Activity之类的。是我哪里搞错了还是什么原因，求大神解释。
接下来就是主文件，继承Activity的

 public class MainActivity extends Activity implements OnClickListener {
    private ProgressBar progressBar;
    private ProgressBarOperation operation;
    private Button btn_start, btn_stop;

    static {
        System.loadLibrary("Pressure");
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        progressBar = (ProgressBar) findViewById(R.id.progressBar);
        operation = new ProgressBarOperation();
        btn_start = (Button)findViewById(R.id.btn_start);
        btn_start.setOnClickListener(this);
        btn_stop = (Button)findViewById(R.id.btn_stop);
        btn_stop.setOnClickListener(this);
    }

    @Override
    public void onClick(View v) {
        if (v.getId() == R.id.btn_start) {
            new Thread(new Runnable() {
                @Override
                public void run() {
                    operation.startMonitor();
                }
            }).start();
        } else if (v.getId() == R.id.btn_stop) {
            operation.stopMonitor();
        }
    }

    public void setPressure(int pressure) {
        Log.e("pressure", "" + pressure);
        progressBar.setProgress(pressure);
    }
}

接下来就是自己编写jni文件了。中间自动生成头文件就不说了
我把整个cpp文件全部贴过来

 #include <jni.h>
#include <stdlib.h>
#include <unistd.h>
#include "com_example_pressuretest_ProgressBarOperation.h"

int getPressure() {
    return rand() % 100;
}

int flag = JNI_TRUE;

JNIEXPORT void JNICALL Java_com_example_pressuretest_ProgressBarOperation_startMonitor(
        JNIEnv *env, jobject obj) {
    while (flag) {
        sleep(1);
        //拿到字节码对象
        jclass clazz = env->FindClass("com/example/pressuretest/MainActivity");
        jmethodID methodId = env->GetMethodID(clazz, "setPressure", "(I)V");
        jobject dpobj= env->NewObject(clazz, methodId);
        env->CallVoidMethod(dpobj, methodId, getPressure());
    }
}

JNIEXPORT void JNICALL Java_com_example_pressuretest_ProgressBarOperation_stopMonitor(
        JNIEnv *env, jobject obj) {
    flag = JNI_FALSE;
}

env->CallVoidMethod(dpobj, methodId, getPressure());这句话一直报错，错误提示也看不来究竟错在哪里

Answer 1

我看错了，这个obj代表的是ProgressBarOperation 的对象，你要想办法把MainActivity 的对象传进来或者可以取到，而不能直接去new一个