Retrieving and Parsing Field Modifiers

There are several modifiers that may be part of a field declaration:

Access modifiers: public, protected, and private
Field-specific modifiers governing runtime behavior: transient and volatile
Modifier restricting to one instance: static
Modifier prohibiting value modification: final
Annotations

The method Field.getModifiers() can be used to return the integer representing the set of declared modifiers for the field. The bits representing the modifiers in this integer are defined in java.lang.reflect.Modifier.

The FieldModifierSpy example illustrates how to search for fields with a given modifier. It also determines whether the located field is synthetic (compiler-generated) or is an enum constant by invoking Field.isSynthetic() and Field.isEnumCostant() respectively.


import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
import static java.lang.System.out;

enum Spy { BLACK , WHITE }

public class FieldModifierSpy {
    volatile int share;
    int instance;
    class Inner {}

    public static void main(String... args) {
	try {
	    Class<?> c = Class.forName(args[0]);
	    int searchMods = 0x0;
	    for (int i = 1; i < args.length; i++) {
		searchMods |= modifierFromString(args[i]);
	    }

	    Field[] flds = c.getDeclaredFields();
	    out.format("Fields in Class '%s' containing modifiers:  %s%n",
		       c.getName(),
		       Modifier.toString(searchMods));
	    boolean found = false;
	    for (Field f : flds) {
		int foundMods = f.getModifiers();
		// Require all of the requested modifiers to be present
		if ((foundMods & searchMods) == searchMods) {
		    out.format("%-8s [ synthetic=%-5b enum_constant=%-5b ]%n",
			       f.getName(), f.isSynthetic(),
			       f.isEnumConstant());
		    found = true;
		}
	    }

	    if (!found) {
		out.format("No matching fields%n");
	    }

        // production code should handle this exception more gracefully
	} catch (ClassNotFoundException x) {
	    x.printStackTrace();
	}
    }

    private static int modifierFromString(String s) {
	int m = 0x0;
	if ("public".equals(s))           m |= Modifier.PUBLIC;
	else if ("protected".equals(s))   m |= Modifier.PROTECTED;
	else if ("private".equals(s))     m |= Modifier.PRIVATE;
	else if ("static".equals(s))      m |= Modifier.STATIC;
	else if ("final".equals(s))       m |= Modifier.FINAL;
	else if ("transient".equals(s))   m |= Modifier.TRANSIENT;
	else if ("volatile".equals(s))    m |= Modifier.VOLATILE;
	return m;
    }
}

Sample output follows:

$ java FieldModifierSpy FieldModifierSpy volatile
Fields in Class 'FieldModifierSpy' containing modifiers: 
volatile share    
[ synthetic=false enum_constant=false ]
$ java FieldModifierSpy Spy public
Fields in Class 'Spy' containing modifiers: 
public BLACK    
[ synthetic=false enum_constant=true ]
WHITE    
[ synthetic=false enum_constant=true ]
$ java FieldModifierSpy FieldModifierSpy\$Inner final
Fields in Class 'FieldModifierSpy$Inner' containing modifiers:  
final this$0   
[ synthetic=true  enum_constant=false ]
$ java FieldModifierSpy Spy private static final
Fields in Class 'Spy' containing modifiers:
private static final $VALUES  
[ synthetic=true  enum_constant=false ]

Notice that some fields are reported even though they are not declared in the original code. This is because the compiler will generate some synthetic fields which are needed during runtime. To test whether a field is synthetic, the example invokes Field.isSynthetic(). The set of synthetic fields is compiler-dependent; however commonly used fields include this$0 for inner classes (i.e. nested classes that are not static member classes) to reference the outermost enclosing class and $VALUES used by enums to implement the implicitly defined static method values(). The names of synthetic class members are not specified and may not be the same in all compiler implementations or releases. These and other synthetic fields will be included in the array returned by Class.getDeclaredFields() but not identified by Class.getField() since synthetic members are not typically public.

Because Field implements the interface java.lang.reflect.AnnotatedElement, it is possible to retrieve any runtime annotation with java.lang.annotation.RetentionPolicy.RUNTIME. For an example of obtaining annotations see the section Examining Class Modifiers and Types.

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