Trail: The Reflection API
Lesson: Classes

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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。

Troubleshooting

The following examples show typical errors which may be encountered when reflecting on classes.

Compiler Warning: "Note: ... uses unchecked or unsafe operations"

When a method is invoked, the types of the argument values are checked and possibly converted. ClassWarning invokes getMethod() to cause a typical unchecked conversion warning:

import java.lang.reflect.Method;

public class ClassWarning {
    void m() {
	try {
	    Class c = ClassWarning.class;
	    Method m = c.getMethod("m");  // warning

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

$ javac ClassWarning.java
Note: ClassWarning.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
$ javac -Xlint:unchecked ClassWarning.java
ClassWarning.java:6: warning: [unchecked] unchecked call to getMethod
  (String,Class<?>...) as a member of the raw type Class
Method m = c.getMethod("m");  // warning
                      ^
1 warning

Many library methods have been retrofitted with generic declarations including several in Class. Since c is declared as a raw type (has no type parameters) and the corresponding parameter of getMethod() is a parameterized type, an unchecked conversion occurs. The compiler is required to generate a warning. (See The Java Language Specification, Java SE 7 Edition, sections Unchecked Conversion and Method Invocation Conversion.)

There are two possible solutions. The more preferable it to modify the declaration of c to include an appropriate generic type. In this case, the declaration should be:

Class<?> c = warn.getClass();

Alternatively, the warning could be explicitly suppressed using the predefined annotation @SuppressWarnings preceding the problematic statement.

Class c = ClassWarning.class;
@SuppressWarnings("unchecked")
Method m = c.getMethod("m");  
// warning gone

Tip: As a general principle, warnings should not be ignored as they may indicate the presence of a bug. Parameterized declarations should be used as appropriate. If that is not possible (perhaps because an application must interact with a library vendor's code), annotate the offending line using @SuppressWarnings.

InstantiationException when the Constructor is Not Accessible

Class.newInstance() will throw an InstantiationException if an attempt is made to create a new instance of the class and the zero-argument constructor is not visible. The ClassTrouble example illustrates the resulting stack trace.

class Cls {
    private Cls() {}
}

public class ClassTrouble {
    public static void main(String... args) {
	try {
	    Class<?> c = Class.forName("Cls");
	    c.newInstance();  // InstantiationException

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

$ java ClassTrouble
java.lang.IllegalAccessException: Class ClassTrouble can not access a member of
  class Cls with modifiers "private"
        at sun.reflect.Reflection.ensureMemberAccess(Reflection.java:65)
        at java.lang.Class.newInstance0(Class.java:349)
        at java.lang.Class.newInstance(Class.java:308)
        at ClassTrouble.main(ClassTrouble.java:9)

Class.newInstance() behaves very much like the new keyword and will fail for the same reasons new would fail. The typical solution in reflection is to take advantage of the java.lang.reflect.AccessibleObject class which provides the ability to suppress access control checks; however, this approach will not work because java.lang.Class does not extend AccessibleObject. The only solution is to modify the code to use Constructor.newInstance() which does extend AccessibleObject.

Tip: In general, it is preferable to use Constructor.newInstance() for the reasons described in the Creating New Class Instances section in the Members lesson.

Additional examples of potential problems using Constructor.newInstance() may be found in the Constructor Troubleshooting section of the Members lesson.

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