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Troubleshooting

The following problems are sometimes encountered by developers when trying to invoke constructors via reflection.

InstantiationException Due to Missing Zero-Argument Constructor

The ConstructorTrouble example illustrates what happens when code attempts to create a new instance of a class using Class.newInstance() and there is no accessible zero-argument constructor:


public class ConstructorTrouble {
    private ConstructorTrouble(int i) {}

    public static void main(String... args){
	try {
	    Class<?> c = Class.forName("ConstructorTrouble");
	    Object o = c.newInstance();  // InstantiationException

        // production code should handle these exceptions more gracefully
	} catch (ClassNotFoundException x) {
	    x.printStackTrace();
	} catch (InstantiationException x) {
	    x.printStackTrace();
	} catch (IllegalAccessException x) {
	    x.printStackTrace();
	}
    }
}

$ java ConstructorTrouble
java.lang.InstantiationException: ConstructorTrouble
        at java.lang.Class.newInstance0(Class.java:340)
        at java.lang.Class.newInstance(Class.java:308)
        at ConstructorTrouble.main(ConstructorTrouble.java:7)

Tip: There a number of different reasons an InstantiationException can occur. In this case, the problem is that the presence of the constructor with an int argument prevents the compiler from generating the default (or zero-argument) constructor and there is no explicit zero-argument constructor in the code. Remember that Class.newInstance() behaves very much like the new keyword and will fail whenever new would fail.

Class.newInstance() Throws Unexpected Exception

The ConstructorTroubleToo example shows an unresolvable problem in Class.newInstance(). Namely, it propagates any exception — checked or unchecked — thrown by the constructor.


import java.lang.reflect.InvocationTargetException;
import static java.lang.System.err;

public class ConstructorTroubleToo {
    public ConstructorTroubleToo() {
 	throw new RuntimeException("exception in constructor");
    }

    public static void main(String... args) {
	try {
	    Class<?> c = Class.forName("ConstructorTroubleToo");
	    // Method propagetes any exception thrown by the constructor
	    // (including checked exceptions).
	    if (args.length > 0 && args[0].equals("class")) {
		Object o = c.newInstance();
	    } else {
		Object o = c.getConstructor().newInstance();
	    }

        // production code should handle these exceptions more gracefully
	} catch (ClassNotFoundException x) {
	    x.printStackTrace();
	} catch (InstantiationException x) {
	    x.printStackTrace();
	} catch (IllegalAccessException x) {
	    x.printStackTrace();
	} catch (NoSuchMethodException x) {
	    x.printStackTrace();
	} catch (InvocationTargetException x) {
	    x.printStackTrace();
	    err.format("%n%nCaught exception: %s%n", x.getCause());
	}
    }
}

$ java ConstructorTroubleToo class
Exception in thread "main" java.lang.RuntimeException: exception in constructor
        at ConstructorTroubleToo.<init>(ConstructorTroubleToo.java:6)
        at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
        at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
        at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
        at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
        at java.lang.Class.newInstance0(Class.java:355)
        at java.lang.Class.newInstance(Class.java:308)
        at ConstructorTroubleToo.main(ConstructorTroubleToo.java:15)

This situation is unique to reflection. Normally, it is impossible to write code which ignores a checked exception because it would not compile. It is possible to wrap any exception thrown by a constructor by using Constructor.newInstance() rather than Class.newInstance().

$ java ConstructorTroubleToo
java.lang.reflect.InvocationTargetException
        at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
        at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
        at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
        at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
        at ConstructorTroubleToo.main(ConstructorTroubleToo.java:17)
Caused by: java.lang.RuntimeException: exception in constructor
        at ConstructorTroubleToo.<init>(ConstructorTroubleToo.java:6)
        ... 5 more


Caught exception: java.lang.RuntimeException: exception in constructor

If an InvocationTargetExceptionis thrown, the method was invoked. Diagnosis of the problem would be the same as if the constructor was called directly and threw the exception that is retrieved by InvocationTargetException.getCause(). This exception does not indicate a problem with the reflection package or its usage.

Tip: It is preferable to use
Constructor.newInstance()
over
Class.newInstance()
because the former API permits examination and handling of arbitrary
exceptions thrown by constructors.



Problems Locating or Invoking the Correct Constructor

 The
ConstructorTroubleAgain
class illustrates various ways in which incorrect code can fail to locate or
invoke the expected constructor.


import java.lang.reflect.InvocationTargetException;
import static java.lang.System.out;

public class ConstructorTroubleAgain {
    public ConstructorTroubleAgain() {}

    public ConstructorTroubleAgain(Integer i) {}

    public ConstructorTroubleAgain(Object o) {
	out.format("Constructor passed Object%n");
    }

    public ConstructorTroubleAgain(String s) {
	out.format("Constructor passed String%n");
    }

    public static void main(String... args){
	String argType = (args.length == 0 ? "" : args[0]);
	try {
	    Class<?> c = Class.forName("ConstructorTroubleAgain");
	    if ("".equals(argType)) {
		// IllegalArgumentException: wrong number of arguments
		Object o = c.getConstructor().newInstance("foo");
	    } else if ("int".equals(argType)) {
		// NoSuchMethodException - looking for int, have Integer
		Object o = c.getConstructor(int.class);
	    } else if ("Object".equals(argType)) {
		// newInstance() does not perform method resolution
		Object o = c.getConstructor(Object.class).newInstance("foo");
	    } else {
		assert false;
	    }

        // production code should handle these exceptions more gracefully
	} catch (ClassNotFoundException x) {
	    x.printStackTrace();
	} catch (NoSuchMethodException x) {
	    x.printStackTrace();
	} catch (InvocationTargetException x) {
	    x.printStackTrace();
	} catch (InstantiationException x) {
	    x.printStackTrace();
	} catch (IllegalAccessException x) {
	    x.printStackTrace();
	}
    }
}


$ java ConstructorTroubleAgain
Exception in thread "main" java.lang.IllegalArgumentException: wrong number of arguments
        at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
        at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
        at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
        at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
        at ConstructorTroubleAgain.main(ConstructorTroubleAgain.java:23)


 An
IllegalArgumentException
is thrown because the zero-argument constructor was requested and an attempt
was made to pass an argument.  The same exception would be thrown if the
constructor was passed an argument of the wrong type.

$ java ConstructorTroubleAgain int
java.lang.NoSuchMethodException: ConstructorTroubleAgain.<init>(int)
        at java.lang.Class.getConstructor0(Class.java:2706)
        at java.lang.Class.getConstructor(Class.java:1657)
        at ConstructorTroubleAgain.main(ConstructorTroubleAgain.java:26)


 This exception may occur if the developer mistakenly believes that
reflection will autobox or unbox types.  Boxing (conversion of a primitive to a
reference type) occurs only during compilation.  There is no opportunity in
reflection for this operation to occur, so the specific type must be used when
locating a constructor.

$ java ConstructorTroubleAgain Object
Constructor passed Object


 Here, it might be expected that the constructor taking a
String
argument would be invoked since newInstance() was invoked
with the more specific String type.  However it is too
late!  The constructor which was found is already the constructor with
an
Object
argument.  newInstance() makes no attempt to do method
resolution; it simply operates on the existing constructor object.

Tip: An important difference between new and
Constructor.newInstance()
is that new performs method argument type checking, boxing, and
method resolution.  None of these occur in reflection, where explicit choices
must be made.


IllegalAccessException When Attempting to Invoke an Inaccessible
Constructor

 An
IllegalAccessException
may be thrown if an attempt is made to invoke a private or otherwise
inaccessible constructor.  The

ConstructorTroubleAccess
example illustrates the resulting stack trace.


import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;

class Deny {
    private Deny() {
	System.out.format("Deny constructor%n");
    }
}

public class ConstructorTroubleAccess {
    public static void main(String... args) {
	try {
	    Constructor c = Deny.class.getDeclaredConstructor();
//  	    c.setAccessible(true);   // solution
	    c.newInstance();

        // production code should handle these exceptions more gracefully
	} catch (InvocationTargetException x) {
	    x.printStackTrace();
	} catch (NoSuchMethodException x) {
	    x.printStackTrace();
	} catch (InstantiationException x) {
	    x.printStackTrace();
	} catch (IllegalAccessException x) {
	    x.printStackTrace();
	}
    }
}


$ java ConstructorTroubleAccess
java.lang.IllegalAccessException: Class ConstructorTroubleAccess can not access a member of class Deny with modifiers "private"
        at sun.reflect.Reflection.ensureMemberAccess(Reflection.java:65)
        at java.lang.reflect.Constructor.newInstance(Constructor.java:505)
        at ConstructorTroubleAccess.main(ConstructorTroubleAccess.java:15)


Tip: An access restriction exists which prevents reflective invocation of
constructors which normally would not be accessible via direct invocation.
(This includes---but is not limited to---private constructors in a separate
class and public constructors in a separate private class.)  However,
Constructor is declared to extend
AccessibleObject
which provides the ability to suppress this check via
AccessibleObject.setAccessible().


        
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Troubleshooting

InstantiationException Due to Missing Zero-Argument Constructor

Class.newInstance() Throws Unexpected Exception

Problems Locating or Invoking the Correct Constructor

IllegalAccessException When Attempting to Invoke an Inaccessible Constructor