Develop Reference

How to kill native threads in Android application

I'm using DDMS to monitor threads in my app, and I see that my app has a bunch of native threads as shown in follow picture. And time to time, the number of native threads increased as user interact with my app, which cause my app sometime does not serve as I expect. Is there anyway to kill these native threads?

There is no such thing as a "native thread" on Android, although some people might use that to refer to threads that are not attached to the VM (which would also make them invisible to DDMS). The threads happen to be executing (or waiting) in native code at the time you did a thread dump, but may spend most of their time executing bytecode. (A list of Dalvik thread states is available here.)
The names of the threads suggests that they were created without being given an explicit name. The one thread with a name, NsdManager probably exists because you're using NsdManager, which "responses to requests from an application are on listener callbacks on a seperate thread" [sic].
It's possible that you can glean some useful information from a stack trace. In DDMS, double-click the thread to get a backtrace. On a rooted device, you can kill -3 <pid> to get a full dump, including native stack frames.
Killing arbitrary threads is not allowed, as they might be holding locks or other resources. If you can determine what is starting them, and that they are unnecessary, you can prevent them from being started in the first place.

Killing an *unresponsive* thread in Android/Java

I know the accepted, correct solutions for gracefully closing a thread.
But assume my game is supposed to be fault-tolerant, and when a new gamegplay is started, it tries to gracefully close the (now-paused) thread of the old gameplay. If it fails to join it / close it (e.g. because the old thread is buggy and is in an infinite loop), it instantiates the new Thread and starts it. But I don't like the fact that the old thread is still there, eating resources.
Is there an accepted way to kill an unresponsive thread without killing the process? It seems to me there isn't, in fact, I read somewhere that a Thread might not react to Thread.stop() either.
So there is no way dealing with a thread in an infinite loop (e.g. due to a bug), is it? Even if it reacts to Thread.stop(), the docs say that Thread.stop() may leave Dalvik VM in an inconsistent state...

If you need this capability, you must design it and implement it. Obviously, if you don't design and implement a graceful way to shut down a thread, then there will be no way to gracefully shut down a thread. There is no generic solution because the solution is application-specific. For example, it depends on what resources the thread might hold and what shared state the thread may hold locks on or have corrupted.
The canonical answer is this: If you need this capability, don't use threads. Use processes.
The core reason is the way threads work. You acquire a lock and then you manipulate shared data. While you're manipulating that shared data, it can enter an inconsistent state. It is the absolute responsibility of a thread to restore the data to a consistent state before releasing the lock. (Consider, for example, deleting an object from a doubly-linked list. You must adjust the forward link or the reverse link first. In between those two operations, the linked-list is in an inconsistent state.)
Say you have this code:
Acquire a lock or enter a synchronized block.
Begin modifying the shared state the lock protects.
Bug
Return the data the lock protects to a consistent state.
Release the lock.
So, now, what do we do? At step 3, the thread holds a lock and it has encountered a bug and triggered an exception. If we don't release the lock it acquired in step 1, every thread that tries to acquire that same lock will wait forever, and we're doomed. If we do release the lock it acquired in step 1, every thread that acquires the lock will then see the inconsistent shared state the thread failed to clean up because it never got to step 4. Either way, we're doomed.
If a thread encounters an exceptional condition the application programmer did not create a sane way to handle, the process is doomed.

Possible states for native threads on Android?

What are all the possible thread states during execution for native (C/C++) threads on an Android device? Are they the same as the Java Thread States? Are they Linux threads? POSIX threads?
Not required, but bonus points for providing examples of what can cause a thread to enter each state.
Edit: As requested, here's the motivation:
I'm designing the interface for a sampling profiler that works with native C/C++ code on Android. The profiler reports will show thread states over time. I need to know what all the states are in order to a) know how many distinct states I will need to possibly visually differentiate, and b) design a color scheme that visually differentiates and groups the desirable states versus the undesirable states.

I've been told that native threads on Android are just lightweight processes. This agrees with what I've found for Linux in general. Quoting this wiki page:
A process (which includes a thread) on a Linux machine can be in any of the following states:
TASK_RUNNING - The process is either executing on a CPU or waiting to be executed.
TASK_INTERRUPTIBLE - The process is suspended (sleeping) until some condition becomes true. Raising a hardware interrupt, releasing a system resource the process is waiting for, or delivering a signal are examples of conditions that might wake up the process (put its state back to TASK_RUNNING). Typically blocking IO calls (disk/network) will result in the task being marked as TASK_INTERRUPTIBLE. As soon as the data it is waiting on is ready to be read an interrupt is raised by the device and the interrupt handler changes the state of the task to TASK_INTERRUPTIBLE. Also processes in idle mode (ie not performing any task) should be in this state.
TASK_UNINTERRUPTIBLE - Like TASK_INTERRUPTIBLE, except that delivering a signal to the sleeping process leaves its state unchanged. This process state is seldom used. It is valuable, however, under certain specific conditions in which a process must wait until a given event occurs without being interrupted. Ideally not too many tasks will be in this state.
For instance, this state may be used when a process opens a device file and the corresponding device driver starts probing for a corresponding hardware device. The device driver must not be interrupted until the probing is complete, or the hardware device could be left in an unpredictable state.
Atomic write operations may require a task to be marked as UNINTERRUPTIBLE
NFS access sometimes results in access processes being marked as UNINTERRUPTIBLE
reads/writes from/to disk can be marked thus for a fraction of a second
I/O following a page fault marks a process UNINTERRUPTIBLE
I/O to the same disk that is being accessed for page faults can result in a process marked as UNINTERRUPTIBLE
Programmers may mark a task as UNINTERRUPTIBLE instead of using INTERRUPTIBLE
TASK_STOPPED - Process execution has been stopped; the process enters this state after receiving a SIGSTOP, SIGTSTP, SIGTTIN, or SIGTTOU signal.
TASK_TRACED - Process execution has been stopped by a debugger.
EXIT_ZOMBIE - Process execution is terminated, but the parent process has not yet issued a wait4() or waitpid() system call. The OS will not clear zombie processes until the parent issues a wait()-like call.
EXIT_DEAD - The final state: the process is being removed by the system because the parent process has just issued a wait4() or waitpid() system call for it. Changing its state from EXIT_ZOMBIE to EXIT_DEAD avoids race conditions due to other threads of execution that execute wait()-like calls on the same process.
Edit: And yet the Dalvik VM Debug Monitor provides different states. From its documentation:
"thread state" must be one of:
1 - running (now executing or ready to do so)
2 - sleeping (in Thread.sleep())
3 - monitor (blocked on a monitor lock)
4 - waiting (in Object.wait())
5 - initializing
6 - starting
7 - native (executing native code)
8 - vmwait (waiting on a VM resource)
"suspended" [a separate flag in the data structure] will be 0 if the thread is running, 1 if not.

If you design a system app that has to work with threads in even more advanced way than usual app, I'd first start by examining what API is available on Android to access threads.
The answer is pthread = POSIX threads, with pthread.h header file, implemented in Bionic C library. So you have starting point for knowing what you can achieve.
Another thing is that Android doesn't implement full pthread interface, only subset needed for Android to run.
More on threads + Bionic here, and how they interact with Java and VM is described here. Also I feel that thread is actually a process, as my code uses setpriority(PRIO_PROCESS, gettid(), pr); to set new thread's priority - I don't recall where I got this info, but this works.
I assume that thread may be in running, finished or blocked (e.g. waiting for mutex) state, but that's my a bit limited knowledge since I never needed other thread state.
Now question is if your app can actually retrieve these states using available API in NDK, and if there're more states, if your users would be really interested to know.
Anyway, you may start by displaying possibly incomplete states of threads, and if your users really care, you'd learn about another states from users' feedback and requests.

Google:
Thread.State BLOCKED The thread is blocked and waiting for a lock.
Thread.State NEW The thread has been created, but has never been started.
Thread.State RUNNABLE The thread may be run.
Thread.State TERMINATED The thread has been terminated.
Thread.State TIMED_WAITING The thread is waiting for a specified amount of time.
Thread.State WAITING The thread is waiting.
These states are not very well explained - I don't see the difference between BLOCKED and WAITING, for example.
Interestingly, there is no 'RUNNING' state - do these devices ever do anything?

How to stop a thread that is suspended on a Bluetooth read

My app displays data from a steady stream of Bluetooth text input.
In Android 2.3.4, if I close the socket the read immediately throws an IO exception. In 2.2 is does only most of the time.
I am trying to stop the reading when onStop() is called. Then in onStart() I reconnect.
Is there a better way to kill the thread that is suspended on an inputStream read that is likely to work over all versions?
Thanks
TomZ

I tried interrupting the task and got bogged down in multiple types of exceptions depending on what it was doing at the time of the interrupt and getting compile errors that I was catching exceptions that it said could not be thrown. Even when I did get some working code, it still had reliability problems on Froyo (Galaxy S - Vibrant).
So I backed up and tried using InputStream.available in a loop with a short sleep and a check of a flag that is set to end the read task (so the task was never suspended except on the sleep). This worked great on various android versions.
It seems the trick is to not externally stop the thread but to let it detect the need to quit and return on it's own.
Seems a bit of a kludge, sort of polling the reads. But the code is now stable and the phone performance does not seem to suffer.

How can i force Android to garbage collect old socket information?

I have project where i'm developing an Android App using a lot of existing C++ code accessed through JNI which opens and closes sockets.
Even though this C++ properly handles the closeing of the sockets it opens, it seems either the Android OS or the JVM keeps around references to those sockets/ports being used until the next GC call.
What happens is if we stop the app and start it again before the next GC call, the app cannot create connections on that same socket/port. If we wait for GC to be called by the OS and then restart the app, it successfully creates the connections.
Is there a way to manually free up a socket from Android's/the JVM's perspective? Perhaps a socket class utility? A manual call to GC?

The operating system's TCP/IP protocol stack holds TCP ports for two minutes after the application closes them. So if you've had a listening socket that has accepted connections, the port will remain unusable for a couple of minutes.
Before you bind the socket to the listening address, call setsockopt() on the socket with option=SO_REUSEADDR and value of 1.

Try
System.gc();
Docs say...
void java.lang.System.gc()
Indicates to the virtual
machine that it would be a good time
to run the garbage collector. Note
that this is a hint only. There is no
guarantee that the garbage collector
will actually be run.