Null pointer exception while retrieving all instances of a class with the BIMserver Java client

问题

I am very new to BIMserver and I am trying to get instances of a specific class of the IFC I have checked in, using the Java client library and IfcModelInterface.

Here is the piece of code :

IfcModelInterface model = client.getModel(project, project.getLastRevisionId(),false, true,true);
Collection<IfcProduct> products = model.getAllWithSubTypes(IfcProduct.class);

The call to getAllWithSubtypes results in a null pointer exception. When I debug it goes to the class where :

public <T extends IdEObject> List<T> getAllWithSubTypes(EClass eClass) {
    if (!loadedClasses.contains(eClass.getName()) && modelState != ModelState.FULLY_LOADED) {

eClass is null and hence I get an exception, I don't understand why?

回答1:

Looking at your stacktrace, I assume this line is Connecting.java:48

Collection<IfcProduct> products = model.getAllWithSubTypes(IfcProduct.class);

This calls the following method (IfcModel.java:310)

public <T extends IdEObject> List<T> getAllWithSubTypes(Class<T> interfaceClass)  {
    return getAllWithSubTypes(packageMetaData.getEClass(interfaceClass));
}

And then we come to the NullPointer when eClass.getName() is called in (ClientIfcModel.java:582)

public <T extends IdEObject> List<T> getAllWithSubTypes(EClass eClass) {
    if (!loadedClasses.contains(eClass.getName()) && modelState != ModelState.FULLY_LOADED) {
    ...
}

You pass in an ordinary Java Class interfaceClass which gets mapped into an EMF EClass in order to retrieve all its instances. This mapping is carried out in packageMetaData.getEClass(interfaceClass). It only works if the Class interfaceClass that you pass in pertains to the same IFC schema version as the model's packageMetaData.

For instance, let's say your requested interfaceClass is org.bimserver.models.ifc4.IfcProduct and your model.getPackageMetaData().getSchema() is Schema.IFC2X3TC1, then the mapping will return an EClass null and you will subsequently see the NullPointer.

To prevent the NullPointer exception, you would have to do a runtime check of the model's schema and only request the instances if the schema is what you expect.

来源：https://stackoverflow.com/questions/52246562/null-pointer-exception-while-retrieving-all-instances-of-a-class-with-the-bimser