Friday, October 12, 2007

Class.forName("fully.qualified.ClassName").newInstance() in GWT

In Java, when we design something to work generically like loading a plugin Class or something like that, we use the classloader to load some arbitrary class and instantiate it using the Class.newInstance() (Assuming that our Class has a Zero argument constructor) or using the constructors through the Class's getConstructor method.

This feature cannot be used in GWT, as you may all know that the client code (JavaScript) is statically generated. There is no Classloader or no Class.newInstance() or no reflection at all in Javascript. So to provide very least feature of instantiating a Class by knowing its classname, here is a tip that you may like.

We may not require all the Class types to be available for dynamic instantiation. So we are gonna mark the required Class types as Instantiable (A marker interface).
Now, we need some factory class which has a method newInstance(String className), using which we can create an instance of specified classname.

1 package name.aanand.gwt.client;



4 * Platform factory to create instance of any {@link Instantiable} type.

5 * @author aanandn

6 *

7 */

8 public interface Factory {

9 Instantiable getInstance(String className);

10 }

Now we are gonna create a Class called ReflectiveFactory which is defined as follows...

1 package name.aanand.gwt.client;



4 /**

5 * Factory class for instantiating any {@link Instantiable} type by providing

6 * the fully qualified Class name.

7 * @author aanandn

8 *

9 */

10 public class ReflectiveFactory implements FactoryWrapper{

11 }

The ReflectiveFactory implements an interface called FactoryWrapper which is again a marker interface to identify the factory classes for which Wrapper implementations or Proxies have to be generated. Now we are ready with our base interfaces. And here begins how its achieved...

First have a singleton class which instantiates our Factory like this:

factory = (Factory) GWT.create(ReflectiveFactory.class);

and return this factory whenever its asked for.

Now you may wonder, how will it work without any real code in the ReflectiveFactory. So here is the explaination of how its gonna work.

  1. We are going to extend the GWT compiler to generate a wrapper or proxy for each of the Classes which implement FactoryWrapper.
  2. The generator should create a new class with name Wrapper, which implements Factory.
  3. In the case of ReflectiveFactory, first we are going to get all the Instantiable types.
  4. And finally generate the method newInstance(String className), conditionally returning one of the Reflectables based on the specified className.
To extend GWTCompiler we need to add an entry in our module XML as follows:

1 <module>
3 -- Inherit the core Web Toolkit stuff. -->
4 <inherits name=''/>
6 -- Specify the app entry point class. -->
7 <entry-point class='name.aanand.gwt.client.MainEntryPoint'/>
<generate-with class="name.aanand.gwt.generator.FactoryGenerator" >
<when-type-assignable class="name.aanand.gwt.client.FactoryWrapper" />

The FactoryGenerator class should extend the abstract class Generator and implement its method
public String generate(TreeLogger logger, GeneratorContext context, String typeName)

Simple implemenation of FactoryGenerator is given below:

1 package name.aanand.gwt.generator;


3 import;


5 import;

6 import;

7 import;

8 import;

9 import;

10 import;

11 import;

12 import;

13 import;


15 public class FactoryGenerator extends Generator {


17 public String generate(TreeLogger logger, GeneratorContext context,

18 String typeName) throws UnableToCompleteException {

19 logger.log(TreeLogger.INFO, "Generating source for " + typeName, null);

20 TypeOracle typeOracle = context.getTypeOracle();


22 JClassType clazz = typeOracle.findType(typeName);

23 if (clazz == null) {

24 logger.log(TreeLogger.ERROR, "Unable to find metadata for type '"

25 + typeName + "'", null);

26 throw new UnableToCompleteException();

27 }


29 try {

30 logger.log(TreeLogger.INFO, "Generating source for "

31 + clazz.getQualifiedSourceName(), null);


33 JClassType reflectableType = typeOracle

34 .getType("name.aanand.gwt.client.Instantiable");

35 SourceWriter sourceWriter = getSourceWriter(clazz, context, logger);

36 if (sourceWriter != null) {

37 sourceWriter.println("public "

38 + reflectableType.getQualifiedSourceName()

39 + " newInstance(String className) {");

40 JClassType[] types = typeOracle.getTypes();

41 int count = 0;

42 for (int i = 0; i < types.length; i++) {

43 if (types[i].isInterface() == null

44 && types[i].isAssignableTo(reflectableType)) {

45 if (count == 0) {

46 sourceWriter.println(" if(\""

47 + types[i].getQualifiedSourceName()

48 + "\".equals(className)) {"

49 + " return new "

50 + types[i].getQualifiedSourceName() + "();"

51 + "}");

52 } else {

53 sourceWriter.println(" else if(\""

54 + types[i].getQualifiedSourceName()

55 + "\".equals(className)) {"

56 + " return new "

57 + types[i].getQualifiedSourceName() + "();"

58 + "}");

59 }

60 count++;

61 }

62 }

63 sourceWriter.println("return null;");

64 sourceWriter.println("}");

65 sourceWriter.commit(logger);

66 logger.log(TreeLogger.INFO, "Done Generating source for "

67 + clazz.getName(), null);

68 return clazz.getQualifiedSourceName() + "Wrapper";

69 }

70 } catch (NotFoundException e) {

71 e.printStackTrace();

72 }

73 return null;


75 }


77 public SourceWriter getSourceWriter(JClassType classType,

78 GeneratorContext context, TreeLogger logger) {


80 String packageName = classType.getPackage().getName();

81 String simpleName = classType.getSimpleSourceName() + "Wrapper";

82 ClassSourceFileComposerFactory composer = new ClassSourceFileComposerFactory(

83 packageName, simpleName);

84 composer.addImplementedInterface("name.aanand.gwt.client.Factory");

85 PrintWriter printWriter = context.tryCreate(logger, packageName,

86 simpleName);

87 if (printWriter == null) {

88 return null;

89 } else {

90 SourceWriter sw = composer.createSourceWriter(context, printWriter);

91 return sw;

92 }

93 }


95 }


Now compile your code with GWTCompiler and start using as shown below:

Factory factory = (Factory) GWT.create(ReflectiveFactory.class);

Thats all folks!


Anonymous said...

So close but no dice! Don't know if you can update for GWT 1.5 (if needed) and give a little more detail in how to get working.

f.rochlitzer said...


thx for this fine code snippets, but there is a little bug:
The Interface Factory has to have a methode newInstance and not getInstance.

In addition there is a problem with GWT 1.5.3. In hosted mode runs the code very well, but in web mode there is always a ClassCastException if I try to instantiate the wrapper:
factory = (Factory) GWT.create(ReflectiveFactory.class);

Did you have any suggestions?


f.rochlitzer said...

Hi folks,

I've solved the problem with the ClassCastException in web mode. The create method has to return always return classType.getQualifiedSourceName() + "Wrapper";
and never null.

If there is already a type defined with the provided name then tryCreate returns null, otherwise a PrintWriter is returned. This means the PrintWriter is intantiated only once by GWT every other call returns null.
So a second call (or in web mode the first call) to create returns no PrintWriter and so the create method returns null and gwt uses the origin class literal in the argument of the create method.
This throws then the ClassCastException in web mode.

See also:
Lombardi Development Blog


Anonymous said...

I can't get it to work. =(

First, where is the code to the Instantiable interface?

Second, where is the code to the FactoryWrapper interface.

I'm trying it on GWT 1.7. Is the original code still valid? I'm getting some errors like this one when running the application:

[ERROR] Line 24: No source code is available for type; did you forget to inherit a required module?

venkat said...

Hi rochlitzer, thanks for for your solution to solve ClassCastException in web mode. It saved me....

Thnx a lot..........


eric73 said...


For ClassCastException

Can you shared the source code of Fixed Generator ?



spark0r said...

I was so over joyed when I found this post. It is exactly what we're looking for, I successfully implemented this solution in that it compiles and the project runs but I'm running into a problem when trying to call the .newInstance() method for a new class. Namely, it doesn't seem to hit the generate() method at all. It just happily returns null and goes on it's merry way. Has anybody else run into this? OP, any ideas?

Thank you so much for the post and I hope that you will be able to shed some light on why the .newInstance() method does not seem to be running.



Sam said...

lIt doesn't work with Google App Engine... or do I something wrong?

It runs with firefox, but not chrome and IE...

it breakts on GWT.create()...