I dont know why I get NullPointerException:
ERROR [AWT-EventQueue-0] 18:21:05,864 repast.simphony.ui.RSApplication - Error while initializing simulation
java.lang.NullPointerException
at intraCity_Simulator.GlobalScheduler.load_data(GlobalScheduler.java:20)
at intraCity_Simulator.Initialization_Builder.build(Initialization_Builder.java:306)
at repast.simphony.dataLoader.engine.ClassNameContextBuilder.build(ClassNameContextBuilder.java:41)
Below is the code. The query works when I put it directly in the initialization (context builder) but when I put this code in another class and call it in the context builder it reports NullPointerException.
public void load_data() {
Context<Object> context = ContextUtils.getContext(this);
Iterable<Object> readers = context.getObjects(DataReader.class);
DataReader this_reader = null;
Query<Object> reader_query = new PropertyEquals<Object>(context, "name", "parcel");
for (Object o : reader_query.query()) {
if (o instanceof DataReader) {
this_reader = (DataReader) o;
}
}
System.out.print(this_reader.getName());
}
when I do this in context builder it reports nullPointerException
GlobalScheduler gs = new GlobalScheduler();
context.add(gs);
gs.load_data();
UPDATE:
I just find the line "Context context = ContextUtils.getContext(this);" is not working. the context is still being null. Why? But I need this line as the query needs the context as a parameter.
However, if I follow your suggestion to pass the context directly into the method load_data() it works.
public void load_data(Context context) {
// Context<Object> context = ContextUtils.getContext(this);
DataReader this_reader = null;
System.out.println("context " + context);
Query<Object> reader_query = new PropertyEquals<Object>(context, "name", "parcel");
for (Object o : reader_query.query()) {
System.out.println(o);
if (o instanceof DataReader) {
this_reader = (DataReader) o;
}
}
System.out.print(this_reader.getName());
}
Why can I not be able to identify the context using "Context context = ContextUtils.getContext(this);" ? This method works if it is called as a method in step() but fails if it is called as method in Context Builder.
What's actually null there? Without that its hard to tell if this is a repast question or just fixing a null pointer error. If its the latter, it should be easy enough to determine what variable is null, and then why it is so.
That said, you probably don't need this line
Context<Object> context = ContextUtils.getContext(this);
if you are calling it in the ContextBuilder -- just pass in the context to load_data(). That would at least avoid the context being null.
Related
Assume I have a class C that holds resources that need to be closed as member variables.
public class C {
private ClosableResource1 closableResource1;
private ClosableResource2 closableResource2;
.....
public C(){
closableResource1 = new ClosableResource1();
closableResource2 = new ClosableResource2();
.....
// some logic that can fail
}
close(){
closableResource1.close()
closableResource2.close()
.....
}
}
If the constructor succeeds I can be sure that close() will be called eventually by some entity manager and all the resources will be freed.
But how can I make sure I close the resources when the constructor fails? The failure can happen because I have additional logic in the constructor that can throw exception or I get some RuntimeException outside of my control?
Some things I though of:
Wrapping the constructor body with a try-catch block. Then, assuming I have a lot of closable members I'll have to have a big if statement in the catch block checking which resources were already initializing and only close them.
Offloading the ClosableResources creation to some init() function. Then I would have to make sure init() succeeded every time I try to use the object.
Is there some elegant solution? Or is this much more implementation specific then that?
You can do something like below:
public class C {
private List<AutoCloseable> closableResources = new ArrayList();
private ClosableResource1 closableResource1;
private ClosableResource2 closableResource2;
.....
public C() {
closableResource1 = new ClosableResource1();
closableResources.add(closableResource1)
closableResource2 = new ClosableResource2();
closableResources.add(closableResource2);
.....
try {
// some logic that can fail
} catch(Exception e) {
close();
}
}
close(){
for (AutoCloseable closableResource : closableResources) {
if (closableResource != null) {
closableResource.close();
}
}
}
}
Surrounding your code with try-catch and closing all your resources in catch is the correct solution here. Also read about method finalize() (Here is one tutorial). In general, I would recommend one method that cleans up all the resources (like you suggested method close(), I would call it though cleanup()) and call that method in your catch section and in your finalize() method
I asked and answered a very similar question here. It is very important that a constructor either succeeds or fails completely i.e. leaving no resources open. In order to achieve that I would follow each resource creation statement by a try-catch block. The catch block closes the resource and rethrows the exception so it is not lost:
public C() {
closableResource1 = new ClosableResource1();
closableResource2 = new ClosableResource2();
try {
// .....
// some logic that can fail and throw MyCheckedException or some RuntimeException
} catch (RuntimeException | MyCheckedException e) {
try {closableResource1.close();} catch (Exception ignore) {}
try {closableResource1.close();} catch (Exception ignore) {}
throw e;
}
}
If creating a resource can fail you need nested try-catch blocks as demonstrated here.
Here's a wild idea: create a class called something like DefusableCloser (that you can "defuse", like an explosive device being made safe):
class DefusableCloser implements AutoCloseable {
boolean active = true;
final AutoCloseable closeable;
DefusableCloser(AutoCloseable closeable) {
this.closeable = closeable;
}
#Override public void close() throws Exception {
if (active) closeable.close();
}
}
Now you can use this in a try-with-resources block:
c1 = new CloseableResource();
try (DefusableCloseable d1 = new DefusableCloseable(c1)) {
c2 = new CloseableResource();
try (DefusableCloseable d2 = new DefusableCloseable(c2)) {
// Do the other stuff which might fail...
// Finally, deactivate the closeables.
d1.active = d2.active = false;
}
}
If execution doesn't reach d1.active = d2.active = false;, the two closeables (or one, if the exception was in creating the second resource) will be closed. If execution does reach that line, they won't be closed and you can use them.
The advantage of doing it like this is that the exceptions will be correctly handled.
Note that the ordering is important: don't be tempted to create the two CloseableResources first, then the two DefusableCloseables: doing that won't handle an exception from creating the second CloseableResource. And don't put the creation of the CloseableResources into the TWR, as that would guarantee their closure.
For closing the resources in your class' close() method, you can also use try-with-resources to ensure that both resources are closed:
try (c1; c2) {}
You don't actually have to declare a new variable in the TWR syntax: you can just effectively say "close the resource for this existing variable afterwards", as shown here.
guys. Today I have done my custom realization for WebDriverEventListener. I need only onException() method which will create screenshot. But I got problem because I am using fluent wait.
new FluentWait<>(webDriver)
.withTimeout(Duration.ofSeconds(10))
.pollingEvery(Duration.ofMillis(500))
.ignoring(NoSuchElementException.class)
.until(someCondition)
So, finally, I have got screen for each ignoring(NoSuchElementException.class) - 20 screenshots for 1 fail ))). Had somebody the such problem or had someone resolve it?
when you use .ignoring(NoSuchElementException.class) you don't avoid that the exception is raised, you are just ignoring that exception. What is happening is that the exception is being raised by your FluentWait, but it is ignored (when you declare .ignoring(NoSuchElementException.class)).
You have three options here:
Capture the screen at the end of your test if the test failed [preferred].
Have a Try-Catch wherever you are using your FluentWait or any other Selenium code.
Use reflection to avoid capture when the event is raised from the method that implements the FluentWait.
This is an idea after what we have discussed:
private void ExceptionThrown(object sender, WebDriverExceptionEventArgs e)
{
if (e.ThrownException is NoSuchElementException)
{
// Get the stack trace from the current exception
StackTrace stackTrace = new StackTrace(e.ThrownException, true);
// Get the method stack frame index.
int stackTraceIndex = stackTrace.FrameCount - 1;
// Get the method name that caused the exception
string methodName = stackTrace.GetFrame(stackTraceIndex).GetMethod().Name;
if(methodName != "MyFindElement")
{
TakeSceenshot();
}
}
else
{
TakeSceenshot();
}
}
// This is an extension method of the DriverHelper interface
public IWebElement MyFindElement(this IWebDriver driver, By by, int timeOut = 0)
{
var wait = new WebDriverWait(driver, TimeSpan.FromSeconds(timeOut));
wait.IgnoreExceptionTypes(typeof(NoSuchElementException));
// I wait until the element exist
IWebElement result = wait.Until(drv => drv.FindElement(by) != null);
// it means that the element doesn't exist, so we throw the exception
if(result == null)
{
MyPersonalException(by);
}
}
// The parameter will help up to generate more accurate log
public void MyPersonalException(By by)
{
throw new NoSuchElementException(by.ToString());
}
This probably require changes in EventFiringWebDriver, because this class is without WebDriverWait instance and events for them. If you want avoid it, create bool variable in your EventFiringWebDriver extended class and check this value in your OnException like:
protected void OnException(WebDriverExceptionEventArgs e) {
if (IsWaitHandler)
return;
Your actions...
}
but this is not perfect solution.
I need to be able to add items to a listbox inside of a thread. Code is below:
1. ref class Work
2. {
3. public:
4. static void RecieveThread()
5. {
6. while (true)
7. {
8. ZeroMemory(cID, 64);
9. ZeroMemory(message, 256);
10. if(recv(sConnect, message, 256, NULL) != SOCKET_ERROR && recv(sConnect, cID, 64, NULL) != SOCKET_ERROR)
11. {
12. ID = atoi(cID);
13. String^ meep = gcnew String(message);
14. lbxMessages->Items->Add(meep);
15. check = 1;
16. }
17. }
18. }
19. };
I get the error
Error: a nonstatic member reference must be relative to a specific object on line 14. Is there any way to get it to let me do that? Because if I try to use String^ meep; outside of that Thread it doesn't contain anything. It works PERFECT when I use it within the thread but not outside of it. I need to be able to add that message to the list-box. If anyone can help I would appreciate it.
You don't show how lbxMessages is defined, but I'm going to assume that it's a non-static data member in the same class. If that's the case, then you need to specify which object you want to access lbxMessages on. The simplest way would be to switch the RecieveThread method to be non-static, then you can access this->lbxMessages.
You didn't say which windowing toolkit you're using, but you probably will need to Invoke back onto the UI thread in order to edit the control.
One way is using System::Thread with a ParameterizedThreadStart delegate, which let you pass objects, in this case lbxMessages.
ParameterizedThreadStart^ threadCallback;
threadCallback = gcnew ParameterizedThreadStart(&Work::ReceiveThread);
Thread^ recvThread = gcnew Thread( threadCallback );
recvThread->Start( lbxMessages );
Your static method for running the thread:
static void RecieveThread(Object^ state)
{
ListBox^ lbxMessages = (ListBox^)state;
//...code
}
But.. . there is another problem. Assuming ListBox is a Win32 control, you can make changes in control only from the thread which it was created. So every time you insert a ListBox item, it must be done from the UI's thread. One way is using a SynchronizationContext object.
// Start the thread
array<Object^>^ args = gcnew array<Object^>(2){
lbxMessages,
SynchronizationContext::Current
}
recvThread->Start( args);
Thread method should be something like this:
static void RecieveThread(Object^ state)
{
array<Object^>^ args = (array<Object^>^)state;
ListBox^ lbxMessages = (ListBox^)args[0];
SynchronizationContext^ ctx = (SynchronizationContext^)args[1];
//...code
String^ meep = gcnew String(message);
ctx->Send(gcnew SendOrPostCallback(&Work::SafeListBoxInsert),
gcnew array<Object^>(2){lbxMessages, meep}
);
}
You will need another method to be called from UI's thread and make the changes.
ref class Work{
//...other methods
static void SafeListBoxInsert(Object^ state)
{
array<Object^>^ args = (array<Object^>^)state;
ListBox^ lst = (ListBox^) args[0];
String^ item = (String^) args[1];
lst->Items->Add(item);
}
}
I'm playing around with editing java.lang.Object for the Java Runtime Environment. I realize that there are probably better ways to do what I want, but that is not what my question is about.
Basically I've added a constructor to java.lang.Object which gets called everytime an object is created. I'm waiting for a certain class to load like so:
public Object() {
if (hookEnabled) {
hookEnabled = false;
objectCount++;
if (objectCount > objectStartCount) {
if (this.getClass() != null) {
String name = this.getClass().getName();
if ((!name.startsWith("java.")) && (!name.startsWith("javax.")) && (!name.startsWith("launcher.")) && (!name.startsWith("sunw.")) && (!name.startsWith("com.sun.")) && (!name.startsWith("sun.")) && (!name.startsWith("org.xml.")) && (!name.startsWith("org.w3c.")) && (!name.startsWith("org.omg.")) && (!name.startsWith("org.ietf."))) {
if (!hasHooked) {
hasHooked = true;
//startup beep
java.awt.Toolkit.getDefaultToolkit().beep();
//load interface
javax.swing.JFrame frame = new javax.swing.JFrame("");
frame.setBounds(0, 0, 400, 400);
frame.setAlwaysOnTop(true);
frame.setVisible(true);
}
}
}
}
hookEnabled = true;
}
}
This works fine. It adds a window to whatever application is being run by the JVM.
However, when making a simple change by moving the JFrame code into a separate class, and calling that call the JVM simply crashes:
public Object() {
if (hookEnabled) {
hookEnabled = false;
objectCount++;
if (objectCount > objectStartCount) {
if (this.getClass() != null) {
String name = this.getClass().getName();
if ((!name.startsWith("java.")) && (!name.startsWith("javax.")) && (!name.startsWith("launcher.")) && (!name.startsWith("sunw.")) && (!name.startsWith("com.sun.")) && (!name.startsWith("sun.")) && (!name.startsWith("org.xml.")) && (!name.startsWith("org.w3c.")) && (!name.startsWith("org.omg.")) && (!name.startsWith("org.ietf."))) {
if (!hasHooked) {
hasHooked = true;
(new tvmh.DFVMH()).setup();
}
}
}
}
hookEnabled = true;
}
}
--
package tvmh;
public class DFVMH {
public void setup() {
//startup beep
java.awt.Toolkit.getDefaultToolkit().beep();
//load interface
javax.swing.JFrame frame = new javax.swing.JFrame("");
frame.setBounds(0, 0, 400, 400);
frame.setAlwaysOnTop(true);
frame.setVisible(true);
}
}
The same happens when I try to create a java.util.Timer object.
Interestingly enough, the above does work if I make DFVMH an inline class (internal class) of java.lang.Object itself.
Could anyone tell me why such behaviour would happen? And is there any way to safely call such custom class?
Tinkering with the innards of the JVM like this is very risky. There are all sorts of hidden dependencies at the low levels of the JVM that can break. JVM bootstrap is a very delicate process.
For instance, the most likely reason you are seeing a crash rather a StackOverflowError is that your change has broken all object construction ... including construction of the error object.
And I suspect that your guard code is ineffective because this.getClass().getName() may be causing a String object to be created. So the fatal recursion happens before you get to your guard.
(Incidentally, your hasHooked flag introduces a race condition.)
My advice is "Don't do it!".
What do you mean by 'it crashes'?.
Isn't it StackOverflowException? Your new tvmh.DFVMH() is actually a constructor all too. So it runs through your 'overriden' Object constructor.
If you already play like this, how about adding the tvmh.DFVMH to stop list of packages/classes?
Just a quick thought: new tvmh.DFVHM() becomes a new object, which also derives from java.lang.Object, meaning your custom constructor code will be run again before the first one has finished. I'm guessing "hasHooked" should guard against that, but how is that variable defined? If that guard doesn't work, this sequence it will recurse infinitely.
If you make DFVMH an inline class it's name will probably start with "java.lang[...]" (it's in java.lang.Object, after all) and will thus not get through the long if statement with all the name.startsWith.
I'm trying to give a short example of IDynamicMetaObjectProvider for the second edition of C# in Depth, and I'm running into issues.
I want to be able to express a void call, and I'm failing. I'm sure it's possible, because if I dynamically call a void method using the reflection binder, all is fine. Here's a short but complete example:
using System;
using System.Dynamic;
using System.Linq.Expressions;
class DynamicDemo : IDynamicMetaObjectProvider
{
public DynamicMetaObject GetMetaObject(Expression expression)
{
return new MetaDemo(expression, this);
}
public void TestMethod(string name)
{
Console.WriteLine(name);
}
}
class MetaDemo : DynamicMetaObject
{
internal MetaDemo(Expression expression, DynamicDemo demo)
: base(expression, BindingRestrictions.Empty, demo)
{
}
public override DynamicMetaObject BindInvokeMember
(InvokeMemberBinder binder, DynamicMetaObject[] args)
{
Expression self = this.Expression;
Expression target = Expression.Call
(Expression.Convert(self, typeof(DynamicDemo)),
typeof(DynamicDemo).GetMethod("TestMethod"),
Expression.Constant(binder.Name));
var restrictions = BindingRestrictions.GetTypeRestriction
(self, typeof(DynamicDemo));
return new DynamicMetaObject(target, restrictions);
}
}
class Test
{
public void Foo()
{
}
static void Main()
{
dynamic x = new Test();
x.Foo(); // Works fine!
x = new DynamicDemo();
x.Foo(); // Throws
}
}
This throws an exception:
Unhandled Exception:
System.InvalidCastException: The
result type 'System.Void' of the
dynamic binding produced by the object
with type 'DynamicDemo' for the binder
'Microsoft.CSharp.RuntimeBinder.CSharpInvokeMemberBinder'
is not compatible with the result type 'System.Object' expected by the
call site.
If I change the method to return object and return null, it works fine... but I don't want the result to be null, I want it to be void. That works fine for the reflection binder (see the first call in Main) but it fails for my dynamic object. I want it to work like the reflection binder - it's fine to call the method, so long as you don't try to use the result.
Have I missed a particular kind of expression I can use as the target?
This is similar to:
DLR return type
You do need to match the return type specified by the ReturnType property. For all of the standard binaries this is fixed to object for almost everything or void (for the deletion operations). If you know you're making a void call I'd suggest wrapping it in:
Expression.Block(
call,
Expression.Default(typeof(object))
);
The DLR used to be quite lax about what it would allow and it would provide some minimal amount of coercion automatically. We got rid of that because we didn't want to provide a set of convensions which may or may not have made sense for each language.
It sounds like you want to prevent:
dynamic x = obj.SomeMember();
There's no way to do that, there'll always be a value returned that the user can attempt to continue to interact with dynamically.
I don't like this, but it seems to work; the real problem seems to be the binder.ReturnType coming in oddly (and not being dropped ("pop") automatically), but:
if (target.Type != binder.ReturnType) {
if (target.Type == typeof(void)) {
target = Expression.Block(target, Expression.Default(binder.ReturnType));
} else if (binder.ReturnType == typeof(void)) {
target = Expression.Block(target, Expression.Empty());
} else {
target = Expression.Convert(target, binder.ReturnType);
}
}
return new DynamicMetaObject(target, restrictions);
Perhaps the callsite expects null to be returned but discards the result - This enum looks interesting, particularly the "ResultDiscarded" flag...
[Flags, EditorBrowsable(EditorBrowsableState.Never)]
public enum CSharpBinderFlags
{
BinaryOperationLogical = 8,
CheckedContext = 1,
ConvertArrayIndex = 0x20,
ConvertExplicit = 0x10,
InvokeSimpleName = 2,
InvokeSpecialName = 4,
None = 0,
ResultDiscarded = 0x100,
ResultIndexed = 0x40,
ValueFromCompoundAssignment = 0x80
}
Food for thought...
UPDATE:
More hints can be gleaned from Microsoft / CSharp / RuntimeBinder / DynamicMetaObjectProviderDebugView which is used (I presume) as a visualizer for debuggers. The method TryEvalMethodVarArgs examines the delegate and creates a binder with the result discarded flag (???)
Type delegateType = Expression.GetDelegateType(list.ToArray());
if (string.IsNullOrEmpty(name))
{
binder = new CSharpInvokeBinder(CSharpCallFlags.ResultDiscarded, AccessibilityContext, list2.ToArray());
}
else
{
binder = new CSharpInvokeMemberBinder(CSharpCallFlags.ResultDiscarded, name, AccessibilityContext, types, list2.ToArray());
}
CallSite site = CallSite.Create(delegateType, binder);
... I'm at the end of my Reflector-foo here, but the framing of this code seems a bit odd since the TryEvalMethodVarArgs method itself expects an object as a return type, and the final line returns the result of the dynamic invoke. I'm probably barking up the wrong [expression] tree.
-Oisin
The C# binder (in Microsoft.CSharp.dll) knows whether or not the result is used; as x0n (+1) says, it keeps track of it in a flag. Unfortunately, the flag is buried inside a CSharpInvokeMemberBinder instance, which is a private type.
It looks like the C# binding mechanism uses ICSharpInvokeOrInvokeMemberBinder.ResultDiscarded (a property on an internal interface) to read it out; CSharpInvokeMemberBinder implements the interface (and property). The job appears to be done in Microsoft.CSharp.RuntimeBinder.BinderHelper.ConvertResult(). That method has code that throws if the aforementioned ResultDiscarded property doesn't return true if the type of the expression is void.
So it doesn't look to me like there's an easy way to tease out the fact that the result of the expression is dropped from the C# binder, in Beta 2 at least.