I have the following SPel expression:
custData.address[0].postcode
The custData is an existing object but the address is an empty list. It is an existing object but it is empty. When I try to set a the post code on this path then I got the
org.springframework.expression.spel.SpelEvaluationException: EL1007E: Property or field 'postcode' cannot be found on null
What I'd need that a new address object will be put to the list and set its postcode attribute.
Is it something that can be done in the SPel expression?
Thanks,
V.
So this basically is a NullPointerException. You need to make sure the object from which you are trying to get a field value exists. SPeL has the special operator '?' to check if the object has value, though I'm not sure if it works for an array, but definitely worth a try. In general expression where some object might be null looks like this:
object?.anotherObject?.field
This makes sure that "object" is not null and if it has value gets "anotherObject" and check if it's not null either, and then gets "field". So try something like this:
custData.address[0]?.postcode
Eventually I ended up using a custom function in spel expression.
#addIfNecessary(custData.address, 0, "uk.co.acme.AddressType").postcode
The user defined function is
import org.springframework.util.ReflectionUtils;
import java.util.List;
public class CustomFunc {
public static Object addIfNecessary(List<Object> list, Integer index, String className) throws IllegalAccessException, InstantiationException, ClassNotFoundException {
Object o = null;
if (list != null) {
if (list.size() <= index || list.get(index) == null) {
list.set(index, Class.forName(className).newInstance());
}
o = list.get(index);
}
return o;
}
}
It is neither nice nor elegant but it works.
Please let me know if you have a more elegant one!
Related
More a general question, but how can I write LINQ Lambda expressions such that they will return a default string or simply an empty string if the LINQ expression fails or returns nothing. In XSLT XPath if a match fails then one just got nothing, and the application did not crash whereas in LINQ one seems to get exceptions.
I use First() and have tried FirstOrDefault().
So example queries may be:
Customers.First(c=>c.id==CustId).Tasks.ToList();
or
Customers.Where(c=>c.id==CustId).ToList();
or
Model.myCustomers.Where(c=>c.id==CustId);
etc.
Whatever the query, if it returns no records or null, then is there a general approach to ensure the query fails gracefully?
Thanks.
There isn't anything elegant built into C# for propagating nulls when you access properties. You could create your own extension methods:
public static class Extensions
{
public static TValue SafeGet<TObject, TValue>(
this TObject obj,
Func<TObject, TValue> propertyAccessor)
{
return obj == null ? default(TValue) : propertyAccessor(obj);
}
public static IEnumerable<T> OrEmpty<T>(this IEnumerable<T> collection)
{
return collection ?? Enumerable.Empty<T>();
}
}
Used like this:
Customers.FirstOrDefault(c => c.id==CustId).SafeGet(c => c.Tasks).OrEmpty().ToList();
Customers.First(c=>c.id==CustId) will crash if there is no matching record.
There are few ways you can try to find it, if you use FirstOrDefault that'll return NULL if no match is found and you can check for NULL.
Or, you can use the .Any syntax which checks if you have any record and returns boolean.
The only query I would expect to throw an exception would be the first one (assuming that Customers is a valid collection and not null itself):
Customers.First(c=>c.id==CustId).Tasks.ToList();
This will throw an exception if there is no customer with an id of CustId (you have some casing issues with your property and variable names).
If you don't wish to throw an exception on no match, then use FirstOrDefault as you mention, and do a null check, e.g:
var customer = Customers.FirstOrDefault(c => c.id == CustId);
if (customer == null)
{
// deal with no match
return;
}
var taskList = customer.Tasks.ToList();
I'm trying to use the IQueryOver interface of a NHibernate session object together with a LINQ expression as a criteria for selecting records in a static class. The LINQ expressions are defined in a mapping class as Expression<Func<T, object>> to get a value for an object T:
public void SearchParameter(Expression<Func<T, object>>)
These parameters get added by extending the mapping class:
public MyMapping : FindMap<MyNHibernateMappedObject>
{
public MyMapping()
{
this.SearchParameter(x => x.SomeColumn);
}
}
My Find class defines static methods for getting the previous and next record of the same type on the time axis. Each search parameter has to be identical in both records.
The Find class gets the search parameters from the mapping configuration and compiles the expressions with .Compile(). So I have the GetQueryWithSearchParameters method:
private static Func<T, object> searchParameter;
...
public static IQueryOver<T, T> GetQueryWithSearchParameters(ISession session, T current)
{
var query = session.QueryOver<T>()
.Where(x => searchParameter(x) == searchParameter(current));
return query;
}
However when building the query, I get the following exception:
System.InvalidOperationException: variable 'x' of type MyNHibernateMappedObject' referenced from scope '', but it is not defined
I don't know exactly what is going on here, but I suspect that x is not available in the delegate somehow. What am I doing wrong here?
session.QueryOver().Where(...) takes an expression, so it's going to try evaluate your expression and translate it to a query - ie. it will try to convert searchParameter(x) == searchParameter(current) into a SQL query, which it won't know how to do.
To get this to work you will need to construct the expression in code (not using a lambda expression). However I think that this is going to be a painful exercise and I think you will find it much easier to build a Criterion and add that to the QueryOver.
I have two classes - MyBaseClass and BaseClassContainer - that are declared like such:
public class MyBaseClass
{
private Guid id;
public Guid ID
{
if (id == Guid.Empty)
{
id = Guid.NewGuid();
}
return id;
}
//...Other Properties omitted for brevity
}
and
public class BaseClassContainer : INotifyPropertyChanged
{
private ObservableCollection<MyBaseClass> baseClasses;
public ObservableCollection<MyBaseClass> BaseClasses
{
//...Omitted for brevity...
}
}
Then in my code I have an ObservableCollection of type BaseClassContainer (BaseClassContainerCollection). What I'm trying to figure out is how can I use LINQ to select a single BaseClassContainer from the ObservableCollection where one of its MyBaseClass.ID matches a specific Guid. The reason I'm using the Single() method is because I know they're all going to be unique.
I've tried the following but it doesn't work:
var result = BaseClassContainerCollection.Single(container => container.BaseClasses.Single(baseClass => baseClass.ID == specificGuid));
I get an error saying: Cannot implicitly convert type 'MyBaseClass' to 'bool'. What am I missing?
Lets break apart your query:
BaseClassContainerCollection.Single(yourPredicate);
Single, as it is used here, basically says "filter BaseClassContainerCollection on this predicate" (a "filter" function that evaluates to true or false for whether or not to include it in the results). Instead of a function that returns true/false, you're saying you want it to evaluate to a MyBaseClass, which doesn't make sense. Your inner call to Single makes sense, because x => x.Id == guid is a function that returns true/false and filters to only those elements that meet the criteria (then states that you know there will only be one of them in the results or else throw an exception).
What you want to do is Select the single MyBaseClass result from the inner query, then call Single on the result (without a predicate) since you know the result should only have one item returned. I believe you're looking for:
BaseClassContainerCollection.Select(container => container.BaseClasses.Single(baseClass => baseClass.ID == specificGuid)).Single();
This question already has answers here:
Validation Error: Value is not valid
(3 answers)
Closed 6 years ago.
NetBeans 7.1.1 JSF2.1
When using converter="convK" attribute in h:selectManyCheckBox it all works well. But I tried to use #FacesConverter(forClass=className.class) form and it keeps giving me "Validation is not Valid" errors. I've tried changing it to forClass=packageName.className.class but no help.
This is converter:
import javax.faces.component.UIComponent;
import javax.faces.context.FacesContext;
import javax.faces.convert.Converter;
import javax.faces.convert.FacesConverter;
#FacesConverter( "convK")
public class KorisnikConverter implements Converter{
#Override
public Object getAsObject(FacesContext context, UIComponent component, String value) {
if (value==null) return value;
if (value.isEmpty()) return value;
for (int i=0; i<Arhiva.getSviKor().size(); i++) {
if (Arhiva.getSviKor().get(i).getUsername().equals(value)) {
return Arhiva.getSviKor().get(i);
}
}
return value;
}
#Override
public String getAsString(FacesContext context, UIComponent component, Object value) {
if (value==null) return "";
if (value instanceof Korisnik) return ((Korisnik)value).getUsername();
return "";
}
}
I have a class called Korisnik which has couple text fields, username is unique one. In my main managing bean I have couple arrayList of those objects. Goal is to use selectManyCheckBox to chose just some of users and put them in a separate arraylist for some other uses. I wanted to push entire objects around (I can always easily work with strings and have object creation and management in my controler beans but wanted to try custom converters to get selectItems to work with objects)
In my class I've overridden equals and hashCode (as there is a lot of talk about custom converters giving blah blah Validation is not valid errors).
#Override
public boolean equals (Object obj) {
if (obj==null) return false;
if (!(obj instanceof Korisnik)) return false;
Korisnik k = (Korisnik)obj;
return (this.username==k.username);
}
#Override
public int hashCode() {
return this.username.hashCode();
}
Edit. When I'm using it as named converter and using said converter only in that one instance with selectManyCheckbox it works fine even without overriding equals and hashCode.
This is checkbox code
<h:selectManyCheckbox value="#{kontrolg.izabrAut}" layout="pageDirection" converter="convK" >
<f:selectItems value="#{kontrolg.moguciAut}" var="it" itemLabel="# {it.ime} #{it.prezime}" itemValue="#{it}"/>
</h:selectManyCheckbox>
What I don't know is whether I'm failing to properly use forClass="whatever" in converter annotation or my converter actually works ok with that one selectManyCheckbox, but when I specify it in forClass form it gets used for all instances of that object and causes some other code that worked nice before adding custom converters to now give "validation is not valid" error?
The value is not valid validation error will be thrown when the equals() method on the selected item has not returned true for any of the available items.
And indeed, your equals() method is broken. The following line is wrong:
return (this.username==k.username);
I'll assume that username is a String, which is an Object. The == compares Objects by reference, not by their value. In other words, when performing == on two Objects, you're basically testing if they point to exactly the same instance. You're not checking if they represent the same value (say, the Object instance's internal representation). You should be using the Object's equals() method instead, the String#equals() method, here's an extract of relevance from its javadoc:
equals
public boolean equals(Object anObject)
Compares this string to the specified object. The result is true if and only if the argument is not null and is a String object that represents the same sequence of characters as this object.
The == is only applicable when comparing primitives like boolean, int, long, etc or when testing for null.
So, to fix your problem, replace the wrong line by the following line:
return username.equals(k.username);
Or, when they can possibly be null:
return (username == null) ? (k.username == null) : username.equals(k.username);
See also:
Right way to implement equals contract
I have a Linq query which I am selecting into a string, of course a string can contain null!
So is there a way I can throw an exception within my Linq query, if I detect a null?
Can I decorate my class with an attribute that won't let it allow null?
I would like to wrap my Linq query in a try catch, and as soon as a null is detected then it would enter the catch, and I can handle it.
Edit
Here's my Linq query, it's quite simple currently. I am going to extend it, but this shows the basic shape:
var localText = from t in items select new Items { item = t.name }
Basically item is set to t.name, t.name is a string so it could be empty / null is this perfectly legal as its a string and strings can hold NULL.
So if it returns NULL then I need to throw an exception. Actually it would be handy to be able to throw an exception is NULL or empty.
I seemed to remember some kind of Attributes that can be set on top of properties that says "Don't accept null" etc.?
Edit
I think I found it: http://msdn.microsoft.com/en-us/library/system.componentmodel.dataannotations.requiredattribute.aspx
This doesn't allow null or strings so I presume it throws an exception, I have used this with MVC but I am not sure if I can use it with a standard class.
As a string being null isn't particularly exceptional, you could do something like:
var items = myStrings.Where(s => !string.IsNullOrEmpty(s)).Select(s => new Item(s));
UPDATE
If you are reading this data from an XML file, then you should look into LINQ to XML, and also use XSD to validate the XML file rather than throwing exceptions on elements or attributes that don't contain strings.
You could try intentionally generating a NullReferenceException:
try
{
//Doesn't change the output, but throws if that string is null.
myStrings.Select(s=>s.ToString());
}
catch(NullReferenceException ex)
{
...
}
You could also create an extension method you could tack on to a String that would throw if null:
public static void ThrowIfNull(this string s, Exception ex)
{
if(s == null) throw ex;
}
...
myString.ThrowIfNull(new NullReferenceException());
Why do you want to throw an exception in this case? This sounds like throwing the baby out with the bath water for something that should not happen in the first place.
If you just want to detect that there are null/empty items:
int nullCount= items.Count( x=> string.IsNullOrEmpty(x.name));
If you want to filter them out:
var localText = from t in items where !string.IsNullOrEmpty(t.name) select new Items { item = t.name };