« Previous • Trail • Next »

Retrieving and Parsing Method Modifiers

There a several modifiers that may be part of a method declaration:

Access modifiers: public, protected, and private
Modifier restricting to one instance: static
Modifier prohibiting value modification: final
Modifier requiring override: abstract
Modifier preventing reentrancy: synchronized
Modifier indicating implementation in another programming language: native
Modifier forcing strict floating point behavior: strictfp
Annotations

The MethodModifierSpy example lists the modifiers of a method with a given name. It also displays whether the method is synthetic (compiler-generated), of variable arity, or a bridge method (compiler-generated to support generic interfaces).


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

public class MethodModifierSpy {

    private static int count;
    private static synchronized void inc() { count++; }
    private static synchronized int cnt() { return count; }

    public static void main(String... args) {
	try {
	    Class<?> c = Class.forName(args[0]);
	    Method[] allMethods = c.getDeclaredMethods();
	    for (Method m : allMethods) {
		if (!m.getName().equals(args[1])) {
		    continue;
		}
		out.format("%s%n", m.toGenericString());
		out.format("  Modifiers:  %s%n",
			   Modifier.toString(m.getModifiers()));
		out.format("  [ synthetic=%-5b var_args=%-5b bridge=%-5b ]%n",
			   m.isSynthetic(), m.isVarArgs(), m.isBridge());
		inc();
	    }
	    out.format("%d matching overload%s found%n", cnt(),
		       (cnt() == 1 ? "" : "s"));

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

A few examples of the output MethodMdifierSpy produces follow.

$ java MethodModifierSpy java.lang.Object wait
public final native void java.lang.Object.wait(long) throws java.lang.InterruptedException
  Modifiers:  public final native
  [ synthetic=false var_args=false bridge=false ]
public final void java.lang.Object.wait(long,int) throws java.lang.InterruptedException
  Modifiers:  public final
  [ synthetic=false var_args=false bridge=false ]
public final void java.lang.Object.wait() throws java.lang.InterruptedException
  Modifiers:  public final
  [ synthetic=false var_args=false bridge=false ]
3 matching overloads found
$ java MethodModifierSpy java.lang.StrictMath toRadians
public static double java.lang.StrictMath.toRadians(double)
  Modifiers:  public static strictfp
  [ synthetic=false var_args=false bridge=false ]
1 matching overload found
$ java MethodModifierSpy MethodModifierSpy inc
private synchronized void MethodModifierSpy.inc()
  Modifiers:  private synchronized
  [ synthetic=false var_args=false bridge=false ]
1 matching overload found
$ java MethodModifierSpy java.lang.Class getConstructor
public java.lang.reflect.Constructor<T> java.lang.Class.getConstructor(java.lang.Class<T>[]) throws java.lang.NoSuchMethodException,java.lang.SecurityException
  Modifiers:  public transient
  [ synthetic=false var_args=true  bridge=false ]
1 matching overload found
$ java MethodModifierSpy java.lang.String compareTo
public int java.lang.String.compareTo(java.lang.String)
  Modifiers:  public
  [ synthetic=false var_args=false bridge=false ]
public int java.lang.String.compareTo(java.lang.Object)
  Modifiers:  public volatile
  [ synthetic=true  var_args=false bridge=true  ]
2 matching overloads found

Note that Method.isVarArgs() returns true for Class.getConstructor(). This indicates that the method declaration looks like this:

public Constructor<T> getConstructor(Class<?>... parameterTypes)

not like this:

public Constructor<T> getConstructor(Class<?> [] parameterTypes)

Notice that the output for String.compareTo()contains two methods. The method declared in String.java:

public int compareTo(String anotherString);



and a second synthetic or compiler-generated bridge method.
This occurs because
String
implements the parameterized interface
Comparable.

During type erasure, the argument type of the inherited method
Comparable.compareTo()
is changed from java.lang.Object to java.lang.String.
Since the parameter types for the compareTo methods in
Comparable and String no longer match after erasure,
overriding can not occur.  In all other circumstances, this would produce a
compile-time error because the interface is not implemented.  The addition of
the bridge method avoids this problem.


Method
implements
java.lang.reflect.AnnotatedElement.  Thus any runtime annotations with
java.lang.annotation.RetentionPolicy.RUNTIME
may be retrieved.  For an example of obtaining annotations see the section Examining Class Modifiers and Types.


        
            « Previous
            •
            Trail
            •
            Next »