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Creating New Class Instances

There are two reflective methods for creating instances of classes: java.lang.reflect.Constructor.newInstance() and Class.newInstance(). The former is preferred and is thus used in these examples because:

Class.newInstance() can only invoke the zero-argument constructor, while Constructor.newInstance() may invoke any constructor, regardless of the number of parameters.
Class.newInstance() throws any exception thrown by the constructor, regardless of whether it is checked or unchecked. InvocationTargetException.
Class.newInstance() requires that the constructor be visible; Constructor.newInstance() may invoke private constructors under certain circumstances.



 Sometimes it may be desirable to retrieve internal state from an object
which is only set after construction.  Consider a scenario where it is
necessary to obtain the internal character set used by
java.io.Console.  (The Console character set is stored in an private field and is
not necessarily the same as the Java virtual machine default character set
returned by
java.nio.charset.Charset.defaultCharset()).  The
ConsoleCharset
example shows how this might be achieved:


import java.io.Console;
import java.nio.charset.Charset;
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import static java.lang.System.out;

public class ConsoleCharset {
    public static void main(String... args) {
	Constructor[] ctors = Console.class.getDeclaredConstructors();
	Constructor ctor = null;
	for (int i = 0; i < ctors.length; i++) {
	    ctor = ctors[i];
	    if (ctor.getGenericParameterTypes().length == 0)
		break;
	}

	try {
	    ctor.setAccessible(true);
 	    Console c = (Console)ctor.newInstance();
	    Field f = c.getClass().getDeclaredField("cs");
	    f.setAccessible(true);
	    out.format("Console charset         :  %s%n", f.get(c));
	    out.format("Charset.defaultCharset():  %s%n",
		       Charset.defaultCharset());

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


Note: Class.newInstance()
will only succeed if the constructor is has zero arguments and is already
accessible.  Otherwise, it is necessary to use
Constructor.newInstance()
as in the above example.


Example output for a Unix system:

$ java ConsoleCharset
Console charset          :  ISO-8859-1
Charset.defaultCharset() :  ISO-8859-1


Example output for a Windows system:

C:\> java ConsoleCharset
Console charset          :  IBM437
Charset.defaultCharset() :  windows-1252


 Another common application of
Constructor.newInstance()
is to invoke constructors which take arguments.  The
RestoreAliases
example finds a specific single-argument constructor and invokes it:


import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
import static java.lang.System.out;

class EmailAliases {
    private Set<String> aliases;
    private EmailAliases(HashMap<String, String> h) {
	aliases = h.keySet();
    }

    public void printKeys() {
	out.format("Mail keys:%n");
	for (String k : aliases)
	    out.format("  %s%n", k);
    }
}

public class RestoreAliases {

    private static Map<String, String> defaultAliases = new HashMap<String, String>();
    static {
	defaultAliases.put("Duke", "duke@i-love-java");
	defaultAliases.put("Fang", "fang@evil-jealous-twin");
    }

    public static void main(String... args) {
	try {
	    Constructor ctor = EmailAliases.class.getDeclaredConstructor(HashMap.class);
	    ctor.setAccessible(true);
	    EmailAliases email = (EmailAliases)ctor.newInstance(defaultAliases);
	    email.printKeys();

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


This example uses
Class.getDeclaredConstructor()
to find the constructor with a single argument of type
java.util.HashMap.  Note that it is sufficient to pass HashMap.class since the
parameter to any get*Constructor() method requires a class only
for type purposes.  Due to
type erasure, the following expression evaluates to true:

HashMap.class == defaultAliases.getClass()


The example then creates a new instance of the class using this constructor
with
Constructor.newInstance().

$ java RestoreAliases
Mail keys:
  Duke
  Fang


        
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