I want to know that the following code can be used in multi-threaded environment thread safely. I hope to use Task class as an accessing layer.
public class Task {
public static enum TaskList {
TASK_A {
#Override
void doProcess() {
System.out.println("processing task A");
}
},
TASK_B {
#Override
void doProcess() {
System.out.println("processing task B");
}
},
TASK_C {
#Override
void doProcess() {
System.out.println("processing task C");
}
};
abstract void doProcess();
}
}
This is how it is going to be used.
public static void main(String[] args) {
Thread thread = new Thread(new Runnable() {
#Override
public void run() {
//following code will be executed in multiple threads.
Task.TaskList task = Task.TaskList.TASK_A;
task.doProcess();
}
});
thread.start();
}
Using enums has little to do with thread safety.
Your code (as presented here in the question) is thread-safe. But the following isn't:
public class Task {
public static enum TaskList {
TASK_A {
int x = 0;
#Override
void doProcess() {
x++;
System.out.println("processing task A: " + x);
}
};
abstract void doProcess();
}
}
As you see, it being an enum has no bearing on thread safety. What matters is whether the object (enum value in this case) has any internal state that can change without being properly synchronized, and whether everything that the actual code calls is thread-safe as well.
I have following method for test:
public class classToTest{
#Autowired
private Alternator alternator;
public void methodToTest(){
Thread t = new Thread(new Runnable() {
public void run() {
while(true) {
if(alternator.get()) {
System.out.print("Hello");
alternator.set(false);
}
}
}
};
t.start()
}
}
I need to check that was invoked method
alternator.set(false);
How can I do it?
Instead of starting a thread directly, can you pass in an "Executor" instance?
For example...
public class ClassToTest{
#Autowired
private Alternator alternator;
#Autowired #Qualifier("myExecutor")
private java.util.concurrent.Executor executor;
public void methodToTest() {
Runnable runnable = new Runnable() {
public void run() {
while(true) {
if(alternator.get()) {
System.out.print("Hello");
alternator.set(false);
}
}
};
executor.execute(runnable);
}
}
Now you can test this easier...
public class ClassToTestTest {
...
#Before
public void setup() {
alternator = mock(Alternator.class);
executor = mock(Executor.class);
obj = new ClassToTest();
ReflectionTestUtils.setField(obj, "alternator", alternator);
ReflectionTestUtils.setField(obj, "executor", executor);
}
#Test
public void shouldStartRunnable() {
obj.methodToTest();
ArgumentCaptor<Runnable> runnableCaptor = ArgumentCaptor.forClass(Runnable.class);
verify(executor).execute(runnableCaptor.capture());
Runnable runnable = runnableCaptor.getValue();
// Now test your actual "runnable"...
when(alternator.get()).thenReturn(true);
runnable.run();
verify(alternator).set(false);
}
}
(Have not tried to compile this, so I apologise if there are any mistakes!!)
Though Bret's post of passing in an executor is very much recommended, you can use the timeout() mock verification setting to test for asynchronous conditions.
verify(alternator, timeout(500)).set(false);
Note that this will necessarily increase the flakiness of your test (i.e. the likelihood that the test fails when the code passes). With a sensible timeout value, that flakiness should be negligible, but if you're making this a part of your core test infrastructure you may consider refactoring to allow for synchronous execution in the test.
Say I have an Object Foo that wants to get informed by several running instances of a Thread using a listener interface. E.g.
The interface:
public interface ThreadListener {
public void onNewData(String blabla);
}
The class Foo:
public class Foo implements ThreadListener {
public Foo() {
FooThread th1 = new FooThread();
FooThread th2 = new FooThread();
...
th1.addListener(this);
th2.addListener(this);
...
th1.start();
th2.start();
...
}
#Override
public void onNewData(String blabla) {
...
}
}
The Thread:
public FooThread extends Thread {
private ThreadListener listener = null;
public void addListener(ThreadListener listener) {
this.listener = listener;
}
private void informListener() {
if (listener != null) {
listener.onNewData("Hello from " + this.getName());
}
}
#Override
public void run() {
super.run();
while(true) {
informListener();
}
}
}
In the worst case onNewData(..) is invoked by several threads at the same time. What will happen with Foo? Is it going to crash or not?
Your Foo class has no state (fields), so unless it uses external shared resources (e.g. files...) it is thread safe
Starting thread from a constructor is generally a bad idea although in the case of a state-less object, I suppose it is fine
if onNewData does not access shared data it will work as expected, if it does, the outcome will depend on how the method is implemented.
Can anyone suggest to me how I can pass a parameter to a thread?
Also, how does it work for anonymous classes?
You need to pass the parameter in the constructor to the Runnable object:
public class MyRunnable implements Runnable {
public MyRunnable(Object parameter) {
// store parameter for later user
}
public void run() {
}
}
and invoke it thus:
Runnable r = new MyRunnable(param_value);
new Thread(r).start();
For Anonymous classes:
In response to question edits here is how it works for Anonymous classes
final X parameter = ...; // the final is important
Thread t = new Thread(new Runnable() {
p = parameter;
public void run() {
...
};
t.start();
Named classes:
You have a class that extends Thread (or implements Runnable) and a constructor with the parameters you'd like to pass. Then, when you create the new thread, you have to pass in the arguments, and then start the thread, something like this:
Thread t = new MyThread(args...);
t.start();
Runnable is a much better solution than Thread BTW. So I'd prefer:
public class MyRunnable implements Runnable {
private X parameter;
public MyRunnable(X parameter) {
this.parameter = parameter;
}
public void run() {
}
}
Thread t = new Thread(new MyRunnable(parameter));
t.start();
This answer is basically the same as this similar question: How to pass parameters to a Thread object
via constructor of a Runnable or Thread class
class MyThread extends Thread {
private String to;
public MyThread(String to) {
this.to = to;
}
#Override
public void run() {
System.out.println("hello " + to);
}
}
public static void main(String[] args) {
new MyThread("world!").start();
}
This answer comes very late, but maybe someone will find it useful. It is about how to pass a parameter(s) to a Runnable without even declaring named class (handy for inliners):
String someValue = "Just a demo, really...";
new Thread(new Runnable() {
private String myParam;
public Runnable init(String myParam) {
this.myParam = myParam;
return this;
}
#Override
public void run() {
System.out.println("This is called from another thread.");
System.out.println(this.myParam);
}
}.init(someValue)).start();
Of course you can postpone execution of start to some more convenient or appropriate moment. And it is up to you what will be the signature of init method (so it may take more and/or different arguments) and of course even its name, but basically you get an idea.
In fact there is also another way of passing a parameter to an anonymous class, with the use of the initializer blocks. Consider this:
String someValue = "Another demo, no serious thing...";
int anotherValue = 42;
new Thread(new Runnable() {
private String myParam;
private int myOtherParam;
// instance initializer
{
this.myParam = someValue;
this.myOtherParam = anotherValue;
}
#Override
public void run() {
System.out.println("This comes from another thread.");
System.out.println(this.myParam + ", " + this.myOtherParam);
}
}).start();
So all happens inside of the initializer block.
When you create a thread, you need an instance of Runnable. The easiest way to pass in a parameter would be to pass it in as an argument to the constructor:
public class MyRunnable implements Runnable {
private volatile String myParam;
public MyRunnable(String myParam){
this.myParam = myParam;
...
}
public void run(){
// do something with myParam here
...
}
}
MyRunnable myRunnable = new myRunnable("Hello World");
new Thread(myRunnable).start();
If you then want to change the parameter while the thread is running, you can simply add a setter method to your runnable class:
public void setMyParam(String value){
this.myParam = value;
}
Once you have this, you can change the value of the parameter by calling like this:
myRunnable.setMyParam("Goodbye World");
Of course, if you want to trigger an action when the parameter is changed, you will have to use locks, which makes things considerably more complex.
I know that I'm a few years late, but I came across this issue and took an unorthodox approach. I wanted to do it without making a new class, so this is what I came up with:
int x = 0;
new Thread((new Runnable() {
int x;
public void run() {
// stuff with x and whatever else you want
}
public Runnable pass(int x) {
this.x = x;
return this;
}
}).pass(x)).start();
You can either extend the Thread class or the Runnable class and provide parameters as you want. There are simple examples in the docs. I'll port them here:
class PrimeThread extends Thread {
long minPrime;
PrimeThread(long minPrime) {
this.minPrime = minPrime;
}
public void run() {
// compute primes larger than minPrime
. . .
}
}
PrimeThread p = new PrimeThread(143);
p.start();
class PrimeRun implements Runnable {
long minPrime;
PrimeRun(long minPrime) {
this.minPrime = minPrime;
}
public void run() {
// compute primes larger than minPrime
. . .
}
}
PrimeRun p = new PrimeRun(143);
new Thread(p).start();
To create a thread you normally create your own implementation of Runnable. Pass the parameters to the thread in the constructor of this class.
class MyThread implements Runnable{
private int a;
private String b;
private double c;
public MyThread(int a, String b, double c){
this.a = a;
this.b = b;
this.c = c;
}
public void run(){
doSomething(a, b, c);
}
}
Either write a class that implements Runnable, and pass whatever you need in a suitably defined constructor, or write a class that extends Thread with a suitably defined constructor that calls super() with appropriate parameters.
In Java 8 you can use lambda expressions with the Concurrency API & the ExecutorService as a higher level replacement for working with threads directly:
newCachedThreadPool() Creates a thread pool that creates new threads
as needed, but will reuse previously constructed threads when they are
available. These pools will typically improve the performance of programs that execute many short-lived asynchronous tasks.
private static final ExecutorService executor = Executors.newCachedThreadPool();
executor.submit(() -> {
myFunction(myParam1, myParam2);
});
See also executors javadocs.
As of Java 8, you can use a lambda to capture parameters that are effectively final. For example:
final String param1 = "First param";
final int param2 = 2;
new Thread(() -> {
// Do whatever you want here: param1 and param2 are in-scope!
System.out.println(param1);
System.out.println(param2);
}).start();
Parameter passing via the start() and run() methods:
// Tester
public static void main(String... args) throws Exception {
ThreadType2 t = new ThreadType2(new RunnableType2(){
public void run(Object object) {
System.out.println("Parameter="+object);
}});
t.start("the parameter");
}
// New class 1 of 2
public class ThreadType2 {
final private Thread thread;
private Object objectIn = null;
ThreadType2(final RunnableType2 runnableType2) {
thread = new Thread(new Runnable() {
public void run() {
runnableType2.run(objectIn);
}});
}
public void start(final Object object) {
this.objectIn = object;
thread.start();
}
// If you want to do things like setDaemon(true);
public Thread getThread() {
return thread;
}
}
// New class 2 of 2
public interface RunnableType2 {
public void run(Object object);
}
You can derive a class from Runnable, and during the construction (say) pass the parameter in.
Then launch it using Thread.start(Runnable r);
If you mean whilst the thread is running, then simply hold a reference to your derived object in the calling thread, and call the appropriate setter methods (synchronising where appropriate)
There is a simple way of passing parameters into runnables.
Code:
public void Function(final type variable) {
Runnable runnable = new Runnable() {
public void run() {
//Code adding here...
}
};
new Thread(runnable).start();
}
No you can't pass parameters to the run() method. The signature tells you that (it has no parameters). Probably the easiest way to do this would be to use a purpose-built object that takes a parameter in the constructor and stores it in a final variable:
public class WorkingTask implements Runnable
{
private final Object toWorkWith;
public WorkingTask(Object workOnMe)
{
toWorkWith = workOnMe;
}
public void run()
{
//do work
}
}
//...
Thread t = new Thread(new WorkingTask(theData));
t.start();
Once you do that - you have to be careful of the data integrity of the object you pass into the 'WorkingTask'. The data will now exist in two different threads so you have to make sure it is Thread Safe.
One further option; this approach lets you use the Runnable item like an asynchronous function call. If your task does not need to return a result, e.g. it just performs some action you don't need to worry about how you pass back an "outcome".
This pattern lets you reuse an item, where you need some kind of internal state. When not passing parameter(s) in the constructor care is needed to mediate the programs access to parameters. You may need more checks if your use-case involves different callers, etc.
public class MyRunnable implements Runnable
{
private final Boolean PARAMETER_LOCK = false;
private X parameter;
public MyRunnable(X parameter) {
this.parameter = parameter;
}
public void setParameter( final X newParameter ){
boolean done = false;
synchronize( PARAMETER_LOCK )
{
if( null == parameter )
{
parameter = newParameter;
done = true;
}
}
if( ! done )
{
throw new RuntimeException("MyRunnable - Parameter not cleared." );
}
}
public void clearParameter(){
synchronize( PARAMETER_LOCK )
{
parameter = null;
}
}
public void run() {
X localParameter;
synchronize( PARAMETER_LOCK )
{
localParameter = parameter;
}
if( null != localParameter )
{
clearParameter(); //-- could clear now, or later, or not at all ...
doSomeStuff( localParameter );
}
}
}
Thread t = new Thread(new MyRunnable(parameter));
t.start();
If you need a result of processing, you will also need to coordinate completion of MyRunnable when the sub-task finishes. You could pass a call back or just wait on the Thread 't', etc.
Specially for Android
For callback purposes I usually implement my own generic Runnable with input parameter(s):
public interface Runnable<TResult> {
void run(TResult result);
}
Usage is simple:
myManager.doCallbackOperation(new Runnable<MyResult>() {
#Override
public void run(MyResult result) {
// do something with the result
}
});
In manager:
public void doCallbackOperation(Runnable<MyResult> runnable) {
new AsyncTask<Void, Void, MyResult>() {
#Override
protected MyResult doInBackground(Void... params) {
// do background operation
return new MyResult(); // return resulting object
}
#Override
protected void onPostExecute(MyResult result) {
// execute runnable passing the result when operation has finished
runnable.run(result);
}
}.execute();
}
Create a local variable in your class that extends Thread or implements Runnable.
public class Extractor extends Thread {
public String webpage = "";
public Extractor(String w){
webpage = w;
}
public void setWebpage(String l){
webpage = l;
}
#Override
public void run() {// l is link
System.out.println(webpage);
}
public String toString(){
return "Page: "+webpage;
}}
This way, you can pass a variable when you run it.
Extractor e = new Extractor("www.google.com");
e.start();
The output:
"www.google.com"
First I want to point out that other answers are true.
However, using the parameter in the constructor may not be the best idea for all of you.
In many scenarios you will want to use "Anonymous Inner Class", and override the run() method, because defining specific class for every use is painful.
(new MyRunnable(){...})
And at the time you create that Runnable, the parameter may not be available to you to pass it in the constructor. If for example, you pass this object to a method, that will perform some work in separate thread and then call your runnable, applying the result from that work to it.
In that case, using a method like this one:
public MyRunnable withParameter(Object parameter), may turn out to be far more useful choice.
I do not claim that this is the best solution to the problem, but it will get the job done.
I have a thread inside a class like this-
import java.util.Observable;
public class Download extends Observable {
private int state = 0;
private final Thread myThread = new Thread(() -> {
/*
some work to do here
*/
setChanged();
notifyObservers(state);
});
public void download(int state) {
if (!myThread.isAlive()) {
this.state = state;
myThread.start();
}
}
public Thread getThread() {
return myThread;
}
public static void MyMethod() throws InterruptedException {
Download down = new Download();
down.addObserver((Observable ob, Object dat) -> {
System.out.println(ob);
if ((int) dat == 1) {
down.download(2);
} else {
System.out.println("success");
}
});
down.download(1);
down.getThread().join();
}
public static void main() throws InterruptedException {
MyMethod();
}
}
The problem is I never get it to print the "success" message.
I assume, it is because all observers are being notified from inside of MyThread. So when down.download(2) is called from the observer inside MyMethod(), the previous thread is still running and the call is ignored.
How can I notify all observers from the main thread, not from the myThread?
You are calling down.download(2) from within the execution of MyThread, therefore the thread is still alive which means that your download method does nothing because of if(!myThread.isAlive()).
I would recommend you to use the Executor framework and Listenable Futures from Guava instead of creating threads manually. Example code from the Guava wiki:
ListeningExecutorService service =
MoreExecutors.listeningDecorator(Executors.newFixedThreadPool(10));
ListenableFuture<Explosion> explosion = service.submit(new Callable<Explosion>() {
public Explosion call() {
return pushBigRedButton();
}
});
Futures.addCallback(explosion, new FutureCallback<Explosion>() {
// we want this handler to run immediately after we push the big red button!
public void onSuccess(Explosion explosion) {
walkAwayFrom(explosion);
}
public void onFailure(Throwable thrown) {
battleArchNemesis(); // escaped the explosion!
}
});
Note that Futures.addCallback(..) also has an overload which allows you to determine which executor should execute the callback, this seems to be what you want.