Communication between a working thread and UI thread

Some API like requesting user location, needs to be called in the main UI thread. How to send a message from a working thread to main UI thread? Here is an example:

Kotlin:
- Add a button's action in Fragment (config in XML does not work)
- start a new activity from a Fragment

Android, Add Button Dynamically

Kotlin, adding a button dynamically by code to a Fragment.

Android UI, Line Layout in Constrait Layout

This example show 6 line layout containers in a constraint layout, looks like below:

The layout XML is like

Get Thread JNIEnv

This is an example shows how:

JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM *jvm, void *reserved)
{
    cached_jvm = jvm; /* cache the JavaVM pointer */
    return JNI_VERSION_1_2;
}

static void Some_Function()
{
    (*cached_jvm)->AttachCurrentThread(cached_jvm, &env, NULL);
    (*env)->MonitorEnter(env, callbackInst);
    
    jclass cls = (*env)->GetObjectClass(env, callbackInst);
    jmethodID id = (*env)->GetMethodID(env, cls, "callback", "(I)V");
    (*env)->CallVoidMethod(env, callbackInst, id, (int)l);

    (*env)->MonitorExit(env, callbackInst);
}

CheckJNI of NewStringUTF

In file Check dalvik/vm/checkjni.c

This is the function which check the input string of NewStringUTF is a valid UTF8 or not.
If not valid, the check function will abort the VM.
That caused our application got crashed when there are some mal-formated UTF8 string.

/*
* Verify that "bytes" points to valid "modified UTF-8" data.
*/
static void checkUtfString(JNIEnv* env, const char* bytes, bool nullOk,
    const char* func)
{
    const char* origBytes = bytes;
    if (bytes == NULL) {
        if (!nullOk) {
            LOGW("JNI WARNING: unexpectedly null UTF string\n");
            goto fail;
        }
        return;
    }
    while (*bytes != '\0') {
        u1 utf8 = *(bytes++);
        // Switch on the high four bits.
        switch (utf8 >> 4) {
            case 0x00:
            case 0x01:
            case 0x02:
            case 0x03:
            case 0x04:
            case 0x05:
            case 0x06:
            case 0x07: {
                // Bit pattern 0xxx. No need for any extra bytes.
                break;
            }
            case 0x08:
            case 0x09:
            case 0x0a:
            case 0x0b:
            case 0x0f: {
                /*
                 * Bit pattern 10xx or 1111, which are illegal start bytes.
                 * Note: 1111 is valid for normal UTF-8, but not the
                 * modified UTF-8 used here.
                 */
                LOGW("JNI WARNING: illegal start byte 0x%x\n", utf8);
                goto fail;
            }
            case 0x0e: {
                // Bit pattern 1110, so there are two additional bytes.
                utf8 = *(bytes++);
                if ((utf8 & 0xc0) != 0x80) {
                    LOGW("JNI WARNING: illegal continuation byte 0x%x\n", utf8);
                    goto fail;
                }
                // Fall through to take care of the final byte.
            }
            case 0x0c:
            case 0x0d: {
                // Bit pattern 110x, so there is one additional byte.
                utf8 = *(bytes++);
                if ((utf8 & 0xc0) != 0x80) {
                    LOGW("JNI WARNING: illegal continuation byte 0x%x\n", utf8);
                    goto fail;
                }
                break;
            }
        }
    }
    return;
fail:
    LOGW("             string: '%s'\n", origBytes);
    showLocation(dvmGetCurrentJNIMethod(), func);
    abortMaybe();
}

Android CheckJNI

CheckJNI can catch a number of common errors.

A simple way to enable or disable JNI:
adb remount
adb pull /system/build.prop .

delete or add line ro.kernel.android.checkjni=1

adb push build.prop /system/.
adb shell reboot


Reference:
http://android-developers.blogspot.com/2011/07/debugging-android-jni-with-checkjni.html