Get SingularAttribute from JPA metamodel by looping through a list

Question

I'm having a JPA metamodel where I retrieving attributes from.

@StaticMetamodel(Test.class)
public class Test_{
    public static volatile SingularAttribute<Test, Boolean> var1;
    public static volatile SingularAttribute<Test, Boolean> var2;
    public static volatile SingularAttribute<Test, Boolean> var3;
}

I can call and use them manually:

Test_.var1
Test_.var2
Test_.var3

But how can I do that by looping through a list:

List<String> vars = new ArrayList<String>();
vars.add("var1");
vars.add("var2");
vars.add("var3");

for (int i=0; i < vars.size(); i++) {
    String var = vars.get(i).toString();
    System.out.println(var); // prints: var1, var2, var3

    // How? (s. update below)

}

How do I get the attribute from the metamodel using string var.

Update

I tried to use java reflection as suggested by @naXa within the loop but without success:

// suppose to be
System.out.println(Test_.var1); // com.example.entities.Test.var1
System.out.println(Test_.var1.getType()); // boolean

// looping through list
Class object = Test_.class;   
try {
    Field field = object.getDeclaredField(var);    
    field.setAccessible(true); // public static volatile javax.persistence.metamodel.SingularAttribute com.example.entities.Test.var1
    System.out.println(field); // com.example.entities.Test.var1
    System.out.println(field.getType()); // interface javax.persistence.metamodel.SingularAttribute
    // but <field> is still no Expression <Boolean>
} catch(NoSuchFieldException e) {
    System.out.println("Error"); 
}

It doesn't return the correct name nor type (see output comments in code).

Use reflection to [get field value](https://stackoverflow.com/q/13400075/1429387) — naXa stands with Ukraine, Jul 03 '17 at 23:04
@naXa sorry my bad I copied the wrong part for the example... I corrected that. The list is filed! About reflection, that's what I tried (see update). Now I tried to use `getDeclaredField()` without success? Any further hint? — wittich, Jul 04 '17 at 12:15
@fg78nc there is not such a method. But thx anyway, check the correct answer from [@Dark](https://stackoverflow.com/a/44913453/3001970). — wittich, Jul 04 '17 at 21:49
@wittich http://docs.oracle.com/javaee/6/api/javax/persistence/metamodel/Attribute.html#getJavaType() — fg78nc, Jul 05 '17 at 04:50
@fg78nc sorry you're where right, but I tested it and it was not available. — wittich, Jul 05 '17 at 18:17
Anyways the answer from [@Dark](https://stackoverflow.com/a/44913453/3001970) worked out for me. The key was here the `obj instaceof SingularAttribute`. — wittich, Jul 05 '17 at 19:01

score 1 · Accepted Answer · answered Jul 04 '17 at 20:12

In your Reflection code you are using the field directly but that is not the static SingularAttribute (Test_.var1) value you are looking for. You need to get the actual static value using field.get(null)

Field[] fields = Test_.class.getDeclaredFields();
for (Field field : fields) {
    if (Modifier.isPublic(field.getModifiers())) {
        // For Each Public Field
        try {
            Object obj = field.get(null); // Equivalent to Test_.var1 but you need to cast it
            if (obj instanceof SingularAttribute) {
                System.out.println(obj); // Equivalent to Test_.var1
                System.out.println(SingularAttribute.class.cast(obj).getType()); // Equivalent to Test_.var1.getType()
            }
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
    }
}

Thx for the answer, that's what I was looking for. – wittich Jul 04 '17 at 21:04 — wittich, Jul 04 '17 at 21:04

Get SingularAttribute from JPA metamodel by looping through a list

Update

1 Answers1