Trail: The Reflection API
Lesson: Members
Section: Constructors

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~~The Java Tutorials have been written for JDK 8.~~Java教程是为JDK 8编写的。~~Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.~~本页中描述的示例和实践没有利用后续版本中引入的改进，并且可能使用不再可用的技术。
~~See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.~~有关Java SE 9及其后续版本中更新的语言特性的摘要，请参阅Java语言更改。
~~See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.~~有关所有JDK版本的新功能、增强功能以及已删除或不推荐的选项的信息，请参阅JDK发行说明。

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. Constructor.newInstance() always wraps the thrown exception with an 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 a 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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