I have simple Activity that calls AsyncTask, so I print some id's regarding Proces and Thread:
From onCreate android.os.Process.myUid(): 10137
From onCreate android.os.Process.myPid(): 29776
From onCreate android.os.Process.myTid(): 29776
From onCreate Thread.currentThread().getId(): 1
/****************************************************************/
From Async doInBackground android.os.Process.myUid(): 10137
From Async doInBackground android.os.Process.myPid(): 29776
From Async doInBackground android.os.Process.myTid(): 30426
From Async doInBackground Thread.currentThread().getId(): 12556
Uid is same because its app-specific sandbox
Similar with Pid: Each app is one Process
3rd line in onCreate same as Pid because it's the UIThread and in Android OS as based on Linux we know that issue regarding Process is actually Thread etc... And in the Async the ThreadId is different because AsyncTask runs on different Thread rather then the UIThread
The thing I'm struggling to understand is Thread.currentThread().getId(). What I expect is to get same id as Thread.currentThread().getId() for the same execution environment. e.g. for onCreate I want lines 3,4 to be same (29776), and for Async I expect lines 3,4 to be the same (30426). What is going on here?
Thanks,
Very interesting question by the OP and I decided to dig (love open source).
The short answer is: they're different because they're different, because they were never meant to be the same.
Process.myTid() is the linux thread ID
Thread.getId() is a simple sequential long number.
But the short answer is boring, so let's explore where the answer comes from (links in the answer points to the relevant source codes).
In Process.myTid(), you'll see that is simply calls from Os.gettid() that in itself calls a native method on Libcore for that method is below:
public static int gettid() { return Libcore.os.gettid(); }
furthermore the docs for Os.gettid(); you'll find a link to Linux Programmer's Manual
gettid() returns the caller's thread ID (TID). In a single-threaded
process, the thread ID is equal to the process ID (PID, as returned
by getpid(2)). In a multithreaded process, all threads have the same
PID, but each one has a unique TID.
That means, Process.myTid() returns the thread ID as given by the Linux kernel.
On the other hand Thread.getId() is simply returning a long. This long is assigned during init(...) as tid = nextThreadId();. Then the last piece of this puzzle, below is the code for nextThreadId()
/* For generating thread ID */
private static long threadSeqNumber;
private static synchronized long More ...nextThreadID() {
return ++threadSeqNumber;
}
That means, Thread.getId() is simply a "java layer" static long being auto-increment for each thread.
Related
i am new on xamarin android apps.I want to build an app and i have read that connections with database or the execution of queries should happen asynchronously from the main thread otherwise the UIthread will may collapse.I have found 2 ways to do that:
First way:
WebClient client = new WebClient();
Uri uri = new Uri("http://192.168.2.8/CreateUsername.php");
NameValueCollection parameters = new NameValueCollection();
parameters.Add("Name", txtname.text);
client.UploadValuesCompleted += Client_UploadValuesCompleted;
client.UploadValuesAsync(uri,parameters); ---> is this gonna create a new thread and run asynchronously???
(here i found the first way: https://www.youtube.com/watch?v=jF3D__ibrx8 )
SecondWay:
https://developer.xamarin.com/recipes/android/web_services/consuming_services/call_a_rest_web_service/
Both ways are correct?both of ways are gonna create a new thread and run asynchronously?Thanks!!!!
Both examples are ok but I'd usually trust the official documentation by Xamarin first. What might confuse you with the first example is the lack of await keyword when calling the UploadValuesAsync method. Here's what actually happens:
Call to UploadValuesAsync is made from the UI thread.
UI thread blocks until the method returns because it's running synchronously.
However, the method returns very quickly because it launches the upload process asynchronously on another thread internally and doesn't wait for the process to complete.
When the whole upload process is finished, Client_UploadValuesCompleted will be called.
As you can see on the WebClient.UploadValuesAsync page on MSDN, it states that:
These methods do not block the calling thread.
Also, when looking at the source code for UploadValuesAsync, you'll notice the following attribute defined on top:
[HostProtection(ExternalThreading=true)]
which means the following:
Code that exposes external threading creates or manipulates threads other than its own, which might be harmful to the host.
To sum it up, the upload process is handled on another background thread but otherwise, your code will run synchronously. There's a bit more asynchronicity going on in Xamarin's sample.
I'm pretty new to jni.
I need to invoke a native method from java code, which should perform a time-consuming operation asynchronously (in another thread), and at the end it should invoke a java callback. However I need this callback to be invoked in the same java thread that originated the operation. That is, the java application should be single-threaded, and all callbacks should be invoked in the context of this thread.
I've read tutorials on callback invocations, the stuff about keeping global refs to the object, getting the appropriate methodid and invoking it.
I've also found stuff about how to call a java method from another thread (AttachCurrentThread), however this is not what I need. I need to make the java call within the originating thread, so the question is how do I switch to that thread?
For instance, in Win32 I'd use something like PostMessage/PostThreadMessage to invoke a code in the specified thread, assuming it runs the message-loop. There're also alternatives like QueueUserAPC which is applicable if the thread waits for events in an alertable state.
So, what are the options at my disposal? Is there a jni method for posting execution of a java method in the specified thread? Or perhaps a linux equivalent of PostThreadMessage? I can also think about workarounds within the java code, but prefer to solve this on the native side.
Thanks in advance.
Please see a relevant recent discussion: How do I post code to be run on the Android main thread from a separate thread in C++?. The most interesting part is this thread in android-ndk group from 2012.
TL;NR: from JNI_OnLoad() or other call that happens early enough, and comes from the UI thread, you call
pipe2(messagePipe, O_NONBLOCK | O_CLOEXEC);
ALooper_addFd(ALooper_forThread(), messagePipe[0], 0, ALOOPER_EVENT_INPUT, handler, data);
Elsewhere you define a simple handler function:
int handler(int fd, int, void* pHandler) {
int what = 0;
read(fd, &what, sizeof(what));
static_cast<MyHandler *>(pHandler)->handle(what));
return 1;
}
Now the native version of Handler.sendEmptyMessage(), and a close analog of CWinThread::PostThreadMessage() (without wParam and lParam), could then be as simple as
MyHandler::sendEmptyMessage(int what) {
write(messagePipe[1], &what, sizeof(what));
}
For example there is an AsyncTask of a String... parameters , if I make a call like this :
AsyncTask<String, Void, Void> someTask = new myTask(myActivity.this);
someTask.execute(string1 , string2 , string3);
What is the internal order of execution of the doInBackground inside this task : does it treat string1 first then string2 and so on sequencely as they are provided when called , or does it treat the parameters randomly ?
First thing, parameters are not passed randomly. This answer will explain you more about parameters. Also check image from this answer. I am adding same image here for your understanding.
It may be serial on one thread or parallel, it actually depends upon which version of Android OS your app is running. For most of the case it would be serial on one background thread.
This is what google document says :-
Executes the task with the specified parameters. The task returns itself (this) so that the caller can keep a reference to it.
Note: this function schedules the task on a queue for a single background thread or pool of threads depending on the platform version. When first introduced, AsyncTasks were executed serially on a single background thread. Starting with DONUT, this was changed to a pool of threads allowing multiple tasks to operate in parallel. Starting HONEYCOMB, tasks are back to being executed on a single thread to avoid common application errors caused by parallel execution. If you truly want parallel execution, you can use the executeOnExecutor(Executor, Params...) version of this method with THREAD_POOL_EXECUTOR; however, see commentary there for warnings on its use.
This method must be invoked on the UI thread.
Check this link execute (Params... params) it will help you.
Hope it helps,
Thanks.
String... is a "vararg", which in this example converts all individual parameters into a String[], where the entries to the array are in the order they got passed into the method.
So using your example, (String[]) param[0] == string1, param[1] == string2, param[2] == string3 and so forth. This is for the ordering of param entries, as to how each entry in param is used, it depends entirely on your code.
As we know asynctask works as a background thread and returns control to the gui when task is complete and requests through volley can be made in the main activity thread ,
Lets say i assign the JSON response to a string variable upon the completion of the request , the string is initially null , the program further processes that reponse,
the problem that i got was i was thrown a NullPointerException while processing the response string even after the successful response ,
it would be absurd to say but is it possible if the response is slow the program may continue without initializing the string variable ??
It sounds like you are not using a proper callback on your data. In other words, after you start AsyncTask, you have to wait until it is done until you reference global variables that it might impact.
So, if you didn't write a proper callback method, then yes the program will continue. The idea behind AsyncTask that - to let the thread it is called on continue while it does a long process of some kind.
I have a C thread that is making requests and receiving updates from a server. The updates are sent to Java through JNI calls. My problem happens when I'm receiving a player's inventory which can contain up to 100 items (100 responses from the server, I cannot modify this part). Sometimes the problem happens, sometimes not but the bigger the inventory is, the more often I have this issue.
I don't get any exception in the logcat except the following message :
06-10 10:09:46.085: I/Choreographer(23815): Skipped 87 frames! The application may be doing too much work on its main thread.
And then my app closes. I also need to say that even when I comment the lines where I update the UI with runOnUiThread the app crash.
I tried to check if I'm on the UI Thread when I return to Java with JNI but Looper.myLooper() == Looper.getMainLooper() return false.
Anyone experienced the same problem ? Are the C threads related to the main thread somehow ? Thanks
EDIT
When I receive an update from the server the following calls are made :
From a Java Thread (not the UI thread) : call a C function named notifyAll
From notifyAll call a C function named update which will call its equivalent in Java (see code below)
void UpdateListenerWrapper::update(Update& u) {
// Retrieve the current JNIEnv* with the cached JVM
int status;
JNIEnv* env;
bool isAttached = false;
status = gCachedJVM->GetEnv((void **) &env, JNI_VERSION_1_2);
if(status < 0) {
__android_log_print(ANDROID_LOG_ERROR, "UpdateListenerWrapper", "Failed to get JNI environment");
status = gCachedJVM->AttachCurrentThread(&env, NULL);
if(status < 0) {
__android_log_print(ANDROID_LOG_ERROR, "UpdateListenerWrapper", "Failed to attach current thread");
return;
}
isAttached = true;
}
jmethodID update = env->GetMethodID(gClazzUpdateListenerWrapper, "update", "(J)V"); // J stands for Java long type
// Call Java method update from jUpdateListener object
env->CallVoidMethod(jUpdateListener, update, (jlong)(intptr_t)&u); // Pointer as agument, we'll build the Update object in Java
if (isAttached) {
gCachedJVM->DetachCurrentThread();
}
}
I think the problem is at this line gCachedJVM->GetEnv((void **) &env, JNI_VERSION_1_2); and maybe GetEnv return a pointer for the UI thread. Could that be the problem ? How can I fix this ?
The app crash is unrelated to the Choreographer complaints. Those are just a warning that indicates the animation is being starved.
You really want to be viewing the logcat output in a mode that shows thread IDs. I recommend adb logcat -v threadtime from the command line. If you put a log message at the start of your server interaction you can easily see if it's running on the UI thread (thread ID and process ID are the same -- not guaranteed by the system, but reliably true in apps).
Never do network or database I/O on the main thread. If it takes too long, the system gets bored and decides that your app is not responding.
Calling into native code through JNI does not switch you to a different thread. There are not C threads and Java threads, just threads, which can call in and out of code written in C and Java.
Re: question updates...
GetEnv always returns a pointer to data for the current thread. Also, CallVoidMethod always happens in the current thread; even if you passed in the wrong JNIEnv it wouldn't "jump" threads.
The GetMethodID call can be expensive in a class with a lot of methods, so you should try to cache that at the same point where gClassUpdateListenerWrapper is set up. Attaching and detaching the thread from the VM can also be expensive and is something best avoided -- and if you're calling here from a Java method, then by definition it's attached already. I would guess that isAttached is never being set.
That doesn't really explain why the Choreographer is starving though. I think you still need to add a log message to C update() and use logcat -v threadtime to get a sense for what is happening on which threads, and use traceview to see where the time is going.
I found the solution to my problem but it's kind of specific to my app. In the update function (Java side), I had a case which had no break and each updates were triggering new network calls (not in the UI thread). Nasty to find but thanks for your time and your answers, you helped me to solve this :)