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。
A class may be declared with one or more modifiers which affect its runtime behavior:
public, protected, and privateabstractstaticfinalstrictfpNot all modifiers are allowed on all classes, for example an interface cannot be final and an enum cannot be abstract. java.lang.reflect.Modifier contains declarations for all possible modifiers. It also contains methods which may be used to decode the set of modifiers returned by Class.getModifiers().
The ClassDeclarationSpyClass implements the java.lang.reflect.AnnotatedElement interface it is also possible to query the runtime annotations.
import java.lang.annotation.Annotation;
import java.lang.reflect.Modifier;
import java.lang.reflect.Type;
import java.lang.reflect.TypeVariable;
import java.util.Arrays;
import java.util.ArrayList;
import java.util.List;
import static java.lang.System.out;
public class ClassDeclarationSpy {
public static void main(String... args) {
try {
Class<?> c = Class.forName(args[0]);
out.format("Class:%n %s%n%n", c.getCanonicalName());
out.format("Modifiers:%n %s%n%n",
Modifier.toString(c.getModifiers()));
out.format("Type Parameters:%n");
TypeVariable[] tv = c.getTypeParameters();
if (tv.length != 0) {
out.format(" ");
for (TypeVariable t : tv)
out.format("%s ", t.getName());
out.format("%n%n");
} else {
out.format(" -- No Type Parameters --%n%n");
}
out.format("Implemented Interfaces:%n");
Type[] intfs = c.getGenericInterfaces();
if (intfs.length != 0) {
for (Type intf : intfs)
out.format(" %s%n", intf.toString());
out.format("%n");
} else {
out.format(" -- No Implemented Interfaces --%n%n");
}
out.format("Inheritance Path:%n");
List<Class> l = new ArrayList<Class>();
printAncestor(c, l);
if (l.size() != 0) {
for (Class<?> cl : l)
out.format(" %s%n", cl.getCanonicalName());
out.format("%n");
} else {
out.format(" -- No Super Classes --%n%n");
}
out.format("Annotations:%n");
Annotation[] ann = c.getAnnotations();
if (ann.length != 0) {
for (Annotation a : ann)
out.format(" %s%n", a.toString());
out.format("%n");
} else {
out.format(" -- No Annotations --%n%n");
}
// production code should handle this exception more gracefully
} catch (ClassNotFoundException x) {
x.printStackTrace();
}
}
private static void printAncestor(Class<?> c, List<Class> l) {
Class<?> ancestor = c.getSuperclass();
if (ancestor != null) {
l.add(ancestor);
printAncestor(ancestor, l);
}
}
}A few samples of the output follows. User input is in italics.
$ java ClassDeclarationSpy java.util.concurrent.ConcurrentNavigableMap Class: java.util.concurrent.ConcurrentNavigableMap Modifiers: public abstract interface Type Parameters: K V Implemented Interfaces: java.util.concurrent.ConcurrentMap<K, V> java.util.NavigableMap<K, V> Inheritance Path: -- No Super Classes -- Annotations: -- No Annotations --
This is the actual declaration for java.util.concurrent.ConcurrentNavigableMap in the source code:
public interface ConcurrentNavigableMap<K,V> extends ConcurrentMap<K,V>, NavigableMap<K,V>
abstract. The compiler adds this modifier for every interface. Also, this declaration contains two generic type parameters, K and V. The example code simply prints the names of these parameters, but is it possible to retrieve additional information about them using methods in java.lang.reflect.TypeVariable. Interfaces may also implement other interfaces as shown above. $ java ClassDeclarationSpy "[Ljava.lang.String;" Class: java.lang.String[] Modifiers: public abstract final Type Parameters: -- No Type Parameters -- Implemented Interfaces: interface java.lang.Cloneable interface java.io.Serializable Inheritance Path: java.lang.Object Annotations: -- No Annotations --
Since arrays are runtime objects, all of the type information is defined by the Java virtual machine. In particular, arrays implement Cloneable and java.io.Serializable and their direct superclass is always Object.
$ java ClassDeclarationSpy java.io.InterruptedIOException Class: java.io.InterruptedIOException Modifiers: public Type Parameters: -- No Type Parameters -- Implemented Interfaces: -- No Implemented Interfaces -- Inheritance Path: java.io.IOException java.lang.Exception java.lang.Throwable java.lang.Object Annotations: -- No Annotations --
From the inheritance path, it may be deduced that java.io.InterruptedIOException is a checked exception because RuntimeException is not present.
$ java ClassDeclarationSpy java.security.Identity Class: java.security.Identity Modifiers: public abstract Type Parameters: -- No Type Parameters -- Implemented Interfaces: interface java.security.Principal interface java.io.Serializable Inheritance Path: java.lang.Object Annotations: @java.lang.Deprecated()
This output shows that java.security.Identity, a deprecated API, possesses the annotation java.lang.Deprecated. This may be used by reflective code to detect deprecated APIs.
java.lang.annotation.RetentionPolicy of RUNTIME are accessible. Of the three annotations pre-defined in the language @Deprecated, @Override, and @SuppressWarnings only @Deprecated is available at runtime.