Suppose one class has two variables : a and b.
Now in that class I am updating the values of two variables in a Synchnonized block, and again updating the values of those two variables in different method.
Now two threads are accessing this class. Will it cause the data inconsistency error ???
Ex:
class ABC {
int a = 9, b = 18;
public void method1() {
synchronized (this) {
a += 10;
b += 12;
}
}
public void method2() {
a += 18;
b += 23;
}
}
Two threads are accessing this object at a time. What will happen ? Whether data consistency will be occur here or not ??
When one thread is running method2() nothing stops another to go through method1(). Since += is not an atomic operation you are open to concurrency problems.
Related
For example, consider the following C# code:
interface IBase { void f(int); }
interface IDerived : IBase { /* inherits f from IBase */ }
...
void SomeFunction()
{
IDerived o = ...;
o.f(5);
}
I know how to get a MethodDefinition object corresponding to SomeFunction.
I can then loop through MethodDefinition.Instructions:
var methodDef = GetMethodDefinitionOfSomeFunction();
foreach (var instruction in methodDef.Body.Instructions)
{
switch (instruction.Operand)
{
case MethodReference mr:
...
break;
}
yield return memberRef;
}
And this way I can find out that the method SomeFunction calls the function IBase.f
Now I would like to know the declared type of the object on which the function f is called, i.e. the declared type of o.
Inspecting mr.DeclaringType does not help, because it returns IBase.
This is what I have so far:
TypeReference typeRef = null;
if (instruction.OpCode == OpCodes.Callvirt)
{
// Identify the type of the object on which the call is being made.
var objInstruction = instruction;
if (instruction.Previous.OpCode == OpCodes.Tail)
{
objInstruction = instruction.Previous;
}
for (int i = mr.Parameters.Count; i >= 0; --i)
{
objInstruction = objInstruction.Previous;
}
if (objInstruction.OpCode == OpCodes.Ldloc_0 ||
objInstruction.OpCode == OpCodes.Ldloc_1 ||
objInstruction.OpCode == OpCodes.Ldloc_2 ||
objInstruction.OpCode == OpCodes.Ldloc_3)
{
var localIndex = objInstruction.OpCode.Op2 - OpCodes.Ldloc_0.Op2;
typeRef = locals[localIndex].VariableType;
}
else
{
switch (objInstruction.Operand)
{
case FieldDefinition fd:
typeRef = fd.DeclaringType;
break;
case VariableDefinition vd:
typeRef = vd.VariableType;
break;
}
}
}
where locals is methodDef.Body.Variables
But this is, of course, not enough, because the arguments to a function can be calls to other functions, like in f(g("hello")). It looks like the case above where I inspect previous instructions must repeat the actions of the virtual machine when it actually executes the code. I do not execute it, of course, but I need to recognize function calls and replace them and their arguments with their respective returns (even if placeholders). It looks like a major pain.
Is there a simpler way? Maybe there is something built-in already?
I am not aware of an easy way to achieve this.
The "easiest" way I can think of is to walk the stack and find where the reference used as the target of the call is pushed.
Basically, starting from the call instruction go back one instruction at a time taking into account how each one affects the stack; this way you can find the exact instruction that pushes the reference used as the target of the call (a long time ago I wrote something like that; you can use the code at https://github.com/lytico/db4o/blob/master/db4o.net/Db4oTool/Db4oTool/Core/StackAnalyzer.cs as inspiration).
You'll need also to consider scenarios in which the pushed reference is produced through a method/property; for example, SomeFunction().f(5). In this case you may need to evaluate that method to find out the actual type returned.
Keep in mind that you'll need to handle a lot of different cases; for example, imagine the code bellow:
class Utils
{
public static T Instantiate<T>() where T : new() => new T();
}
class SomeType
{
public void F(int i) {}
}
class Usage
{
static void Main()
{
var o = Utils.Instantiate<SomeType>();
o.F(1);
}
}
while walking the stack you'll find that o is the target of the method call; then you'll evaluate Instantiate<T>() method and will find that it returns new T() and knowing that T is SomeType in this case, that is the type you're looking for.
So the answer of Vagaus helped me come up with a working implementation.
I published it on github - https://github.com/MarkKharitonov/MonoCecilExtensions
Included many unit tests, but I am sure I missed some cases.
I have a java thread which is running a path-finding algorithm in a constant while loop. Then, every so often I want to retrieve the most updated path from the thread. However, I am unsure how to do this, and think I might be doing it wrong.
My thread consists of the following code:
public class BotThread extends Thread {
Bot bot;
AStar pathFinder;
Player targetPlayer;
public List<boolean[]> plan;
public BotThread(Bot bot) {
this.bot = bot;
this.plan = new ArrayList<>();
pathFinder = new AStar(bot, bot.getLevelHandler());
}
public void run() {
while (true) {
System.out.println("THREAD RUNNING");
targetPlayer = bot.targetPlayer;
plan = pathFinder.optimise(targetPlayer);
}
}
public boolean[] getNextAction() {
return plan.remove(0);
}
}
I then create an object of BotThread, and call start(). Then when I call getNextAction() on the thread, I seem to receive a null pointer. Is this because I am not able to call another method on the thread whilst it is in the main loop? How should I do this properly?
This is because you are not giving enough time to thread to initialise plan Arraylist. You need to add sleeping time to the threads. Something like this while calling BotThread class from main:
int num_threads = 8;
BotThread myt[] = new BotThread[num_threads];
for (int i = 0; i < num_threads; ++i) {
myt[i] = new BotThread();
myt[i].start();
Thread.sleep(1000);
myt[i].getNextAction();
}
I am trying to get multithreading more unraveled in my head. I made these three classes.
A global variable class
public partial class globes
{
public bool[] sets = new bool[] { false, false, false };
public bool boolChanged = false;
public string tmpStr = string.Empty;
public int gcount = 0;
public bool intChanged = false;
public Random r = new Random();
public bool gDone = false;
public bool first = true;
}
Drop in point
class Driver
{
static void Main(string[] args)
{
Console.WriteLine("start");
globes g = new globes();
Thread[] threads = new Thread[6];
ParameterizedThreadStart[] pts = new ParameterizedThreadStart[6];
lockMe _lockme = new lockMe();
for (int b = 0; b < 3; b++)
{
pts[b] = new ParameterizedThreadStart(_lockme.paramThreadStarter);
threads[b] = new Thread(pts[b]);
threads[b].Name = string.Format("{0}", b);
threads[b].Start(b);
}
}
}
And then my threading class
class lockMe
{
#region Fields
private string[] words = new string[] {"string0", "string1", "string2", "string3"};
private globes g = new globes();
private object myKey = new object();
private string[] name = new string[] { String.Empty, String.Empty, String.Empty };
#endregion
#region methods
// first called for all threads
private void setName(Int16 i)
{
Monitor.Enter(myKey);
{
try
{
name[i] = string.Format("{0}:{1}", Thread.CurrentThread.Name, g.r.Next(100, 500).ToString());
}
finally
{
Monitor.PulseAll(myKey);
Monitor.Exit(myKey);
}
}
}
// thread 1
private void changeBool(Int16 a)
{
Monitor.Enter(myKey);
{
try
{
int i = getBools();
//Thread.Sleep(3000);
if (g.gcount > 5) { g.gDone = true; return; }
if (i == 3) resets();
else { for (int x = 0; x <= i; i++) { g.sets[x] = true; } }
Console.WriteLine("Thread {0} ran through changeBool()\n", name[a]);
}
finally
{
Monitor.PulseAll(myKey);
Monitor.Exit(myKey);
}
}
}
// thread 2
private void changeInt(Int16 i)
{
Monitor.Enter(myKey);
{
try
{
g.gcount++;
//Thread.Sleep(g.r.Next(1000, 3000));
Console.WriteLine("Thread {0}: Count is now at {1}\n", name[i], g.gcount);
}
finally
{
Monitor.PulseAll(myKey);
Monitor.Exit(myKey);
}
}
}
// thread 3
private void printString(Int16 i)
{
Monitor.Enter(myKey);
{
try
{
Console.WriteLine("...incoming...");
//Thread.Sleep(g.r.Next(1500, 2500));
Console.WriteLine("Thread {0} printing...{1}\n", name[i], words[g.r.Next(0, 3)]);
}
finally
{
Monitor.PulseAll(myKey);
Monitor.Exit(myKey);
}
}
}
// not locked- called from within a locked peice
private int getBools()
{
if ((g.sets[0] == false) && (g.sets[1] == false) && (g.sets[2] == false)) return 0;
else if ((g.sets[0] == true) && (g.sets[1] == false) && (g.sets[2] == false)) return 1;
else if ((g.sets[2] == true) && (g.sets[3] == false)) return 2;
else if ((g.sets[0] == true) && (g.sets[1] == true) && (g.sets[2] == true)) return 3;
else return 99;
}
// should not need locks- called within locked statement
private void resets()
{
if (g.first) { Console.WriteLine("FIRST!!"); g.first = false; }
else Console.WriteLine("Cycle has reset...");
}
private bool getStatus()
{
bool x = false;
Monitor.Enter(myKey);
{
try
{
x = g.gDone;
}
finally
{
Monitor.PulseAll(myKey);
Monitor.Exit(myKey);
}
}
return x;
}
#endregion
#region Constructors
public void paramThreadStarter(object starter)
{
Int16 i = Convert.ToInt16(starter);
setName(i);
do
{
switch (i)
{
default: throw new Exception();
case 0:
changeBool(i);
break;
case 1:
changeInt(i);
break;
case 2:
printString(i);
break;
}
} while (!getStatus());
Console.WriteLine("fin");
Console.ReadLine();
}
#endregion
}
So I have a few questions. The first- is it better to have my global class set like this? Or should I be using a static class with properties and altering them that way? Next question is, when this runs, at random one of the threads will run, pulse/exit the lock, and then step right back in (sometimes like 5-10 times before the next thread picks up the lock). Why does this happen?
Each thread is given a certain amount of CPU time, I doubt that one particular thread is getting more actual CPU time over the others if you are locking all the calls in the same fashion and the thread priorities are the same among the threads.
Regarding how you use your global class, it doesn't really matter. The way you are using it wouldn't change it one way or the other. Your use of globals was to test thread safety, so when multiple threads are trying to change shared properties all that matters is that you enforce thread safety.
Pulse might be a better option knowing that only one thread can actually enter, pulseAll is appropriate when you lock something because you have a task to do, once that task is complete and won't lock the very next time. In your scenario you lock every time so doing a pulseAll is just going to waste cpu because you know that it will be locked for the next request.
Common example of when to use static classes and why you must make them thread safe:
public static class StoreManager
{
private static Dictionary<string,DataStore> _cache = new Dictionary<string,DataStore>(StringComparer.OrdinalIgnoreCase);
private static object _syncRoot = new object();
public static DataStore Get(string storeName)
{
//this method will look for the cached DataStore, if it doesn't
//find it in cache it will load from DB.
//The thread safety issue scenario to imagine is, what if 2 or more requests for
//the same storename come in? You must make sure that only 1 thread goes to the
//the DB and all the rest wait...
//check to see if a DataStore for storeName is in the dictionary
if ( _cache.ContainsKey( storeName) == false )
{
//only threads requesting unknown DataStores enter here...
//now serialize access so only 1 thread at a time can do this...
lock(_syncRoot)
{
if (_cache.ContainsKey(storeName) == false )
{
//only 1 thread will ever create a DataStore for storeName
DataStore ds = DataStoreManager.Get(storeName); //some code here goes to DB and gets a DataStore
_cache.Add(storeName,ds);
}
}
}
return _cache[storeName];
}
}
What's really important to see is that the Get method only single threads the call when there is no DataStore for the storeName.
Double-Check-Lock:
You can see the first lock() happens after an if, so imagine 3 threads simultaneously run the if ( _cache.ContainsKey(storeName) .., now all 3 threads enter the if. Now we lock so that only 1 thread can enter, now we do the same exact if statement, only the very first thread that gets here will actually pass this if statement and get the DataStore. Once the first thread .Add's the DataStore and exits the lock the other 2 threads will fail the second check (double check).
From that point on any request for that storeName will get the cached instance.
So we single threaded our application only in the spots that required it.
How can I distribute the operations, say duplicate the items\actions sent in one pipe to various different pipes which can access the original pipe?
Say I have Parent thread is "Pthread", I want to link it to 4 or 5 child threads, Just like a binary tree. Any operations performed on "Pthread" should be distributed to all the child threads(Something similar to what ESB does in the SOA architecture).
Like A+B should be sent in all the 5 threads\pipes at the same time and processed.
Is there a way to do this?
public class MainThreadEntry {
public void ThreadCreationMethod()
{
List<Future<Object>> listOfResult = null; // listOfResult is list of Integer objects as a result of computation by different threads
ExecutorService executor = Executors.newFixedThreadPool(5); // no of threads to create from main thread
List<EachThreadComputation> list = new ArrayList<MainThreadEntry .EachThreadComputation>();
for (int i = 0; i < 5; i++) {
EachThreadComputation separeateComputaionInnerClass = new EachThreadComputation(1,2); // innerClass Created For Ecah Thread 1,2 parameter can be dynamic
list.add(separeateComputaionInnerClass);
}
try {
listOfResult = executor.invokeAll(list); // call on different threads with 5 separate executionpath for computation
} catch (InterruptedException e) {
}
}
private class EachThreadComputation implements Callable<Object>{
private int A;
private int B;
EachThreadComputation(int A,int B) {
this.A = A;
this.B = B;
}
#Override
public Object call() throws Exception {
return (Integer)A+B
}
}}
I ran into a rather odd closure issue related to spock unit testing and wondered if anyone could explain this.
If we imagine a dao, model, and service as follows:
interface CustomDao {
List<Integer> getIds();
Model getModelById(int id);
}
class CustomModel {
int id;
}
class CustomService {
CustomDao customDao
public List<Object> createOutputSet() {
List<Model> models = new ArrayList<Model>();
List<Integer> ids = customDao.getIds();
for (Integer id in ids) {
models.add(customDao.getModelById(id));
}
return models;
}
}
I would like to unit test the CustomService.createOutputSet. I have created the following specification:
class TestSpec extends Specification {
def 'crazy closures'() {
def mockDao = Mock(CustomDao)
def idSet = [9,10]
given: 'An initialized object'
def customService = new CustomService
customService.customDao = mockDao
when: 'createOutput is called'
def outputSet = customService.createOutputSet()
then: 'the following methods should be called'
1*mockDao.getIds() >> {
return idSet
}
for (int i=0; i<idSet.size(); i++) {
int id = idSet.get(i)
1*mockDao.getModelById(idSet.get(i)) >> {
def tmp = new Model()
int tmpId = id // idSet.get(i)
return tmp
}
}
and: 'each compute package is accurate'
2 == outputSet.size()
9 == outputSet.get(0).getId()
10 == outputSet.get(1).getId()
}
}
Notice that in here I test two things. First, I initialize the dao with my mock, verify that the daos are correctly called and return the proper data, and then I verify that I get the proper output (i.e. "and:").
The tricky part is the for loop, in which I wanted to return models from the mock dao that are related to the method parameter. In the above example, if I use a simple for (__ in idSet), the models only return with id 10: outputSet.get(0).getId() == outputSet.get(1).getId() == 10. If I use the traditional for loop, and set the model with idSet.get(i), I get an IndexOutOfBoundsException . The only way to make this work is by retrieving the value in a local variable (id) and setting with variable, as above.
I know this is related to groovy closures and I suspect that spock captures the mock calls into a set of closures before executing them, which means that the model creation depends on the outer state of the closure. I understand why I would get the IndexOutOfBoundsException, but I don't understand why int id = idSet.get(i) is captured by the closure whereas i is not.
What is the difference?
Note: this is not the live code but rather simplified to demonstrate the crux of my challenge. I would not and do not make two subsequent dao calls on getIds() and getModelById().
While stubbing getModelById by a closure, the arguments to the closure has to match with that of the method. If you try something like below, you would not need the local variable id inside for anymore.
for (int i=0; i<idSet.size(); i++) {
//int id = idSet.get(i)
mockDao.getModelById(idSet.get(i)) >> {int id ->
def tmp = new Model()
tmp.id = id // id is closure param which represents idSet.get(i)
return tmp
}
}
Simplified version would be to use each
idSet.each {
mockDao.getModelById(it) >> {int id ->
def tmp = new Model()
tmp.id = id // id is closure param which represents idSet.get(i)
tmp
}
}
Do we need to worry about how many times method is called if it is being stubbed?
Accessing mutable local variables from a closure whose execution is deferred is a common source of errors not specific to Spock.
I don't understand why int id = idSet.get(i) is captured by the closure whereas i is not.
The former gives rise to a separate hoisted variable per iteration whose value is constant. The latter gives rise to a single hoisted variable whose value changes over time (and before the result generator executes).
Instead of solving the problem by introducing a temporary variable, a better solution (already given by #dmahapatro) is to declare an int id -> closure parameter. If it's deemed good enough to stub the calls without enforcing them, the loop can be omitted altogether. Yet another potential solution is to construct the return values eagerly:
idSet.each { id ->
def model = new Model()
model.id = id
1 * mockDao.getModelById(id) >> model
}