Pros and Cons:
Pros:
1, reduce configuration xml files
2, readable by self-documenting
Cons:
1, it adds deployment context to classes, which should be
generic enough.
2, interfere with design principles such as IOC and
dependency injection, because you need to introduce imports
Usage (annotation works only when handled by related
annotation processor):
- Information
for the compiler — Annotations can be used by the compiler to detect
errors or suppress warnings.
- Compiler-time
and deployment-time processing — Software tools can process annotation
information to generate code, XML files, and so forth.
- Runtime
processing — Some annotations are available to be examined at runtime.
1 import java.io.IOException;
2 import java.io.PrintStream;
3 import java.lang.reflect.AnnotatedElement;
4 import java.lang.annotation.Annotation;
5
6 import java.util.Date;
7 import java.lang.annotation.Documented;
8 import java.lang.annotation.Inherited;
9 import java.lang.annotation.Retention;
10 import java.lang.annotation.RetentionPolicy;
11
12 public class ReflectionTester {
13
14 public ReflectionTester() {
15 }
16
17 public void testAnnotationPresent(PrintStream out) throws IOException {
18 Class c = Super.class;
19 boolean inProgress = c.isAnnotationPresent(InProgress.class);
20 if (inProgress) {
21 out.println("Super is In Progress");
22 } else {
23 out.println("Super is not In Progress");
24 }
25 }
26
27 public void testInheritedAnnotation(PrintStream out) throws IOException {
28 Class c = Sub.class;
29 boolean inProgress = c.isAnnotationPresent(InProgress.class);
30 if (inProgress) {
31 out.println("Sub is In Progress");
32 } else {
33 out.println("Sub is not In Progress");
34 }
35 }
36
37 public void testGetAnnotation(PrintStream out)
38 throws IOException, NoSuchMethodException {
39
40 Class c = AnnotationTester.class;
41 AnnotatedElement element = c.getMethod("calculateInterest",
42 float.class, float.class);
43
44 GroupTODO groupTodo = element.getAnnotation(GroupTODO.class);
45 String assignedTo = groupTodo.assignedTo();
46
47 out.println("TODO Item on Annotation Tester is assigned to: '" +
48 assignedTo + "'");
49 }
50
51 public void printAnnotations(AnnotatedElement e, PrintStream out)
52 throws IOException {
53
54 out.printf("Printing annotations for '%s'%n%n", e.toString());
55
56 Annotation[] annotations = e.getAnnotations();
57 for (Annotation a : annotations) {
58 out.printf(" * Annotation '%s' found%n",
59 a.annotationType().getName());
60 }
61 }
62
63 public static void main(String[] args) {
64 try {
65 ReflectionTester tester = new ReflectionTester();
66
67 tester.testAnnotationPresent(System.out);
68 tester.testInheritedAnnotation(System.out);
69
70 tester.testGetAnnotation(System.out);
71
72 Class c = AnnotationTester.class;
73 AnnotatedElement element = c.getMethod("calculateInterest",
74 float.class, float.class);
75 tester.printAnnotations(element, System.out);
76 } catch (Exception e) {
77 e.printStackTrace();
78 }
79 }
80 }
81
82 class Sub extends Super {
83
84 public void print(PrintStream out) throws IOException {
85 out.println("Sub printing
");
86 }
87 }
88
89 @InProgress class Super {
90
91 public void print(PrintStream out) throws IOException {
92 out.println("Super printing");
93 }
94 }
95
96 @Documented
97 @Retention(RetentionPolicy.RUNTIME)
98 @interface GroupTODO {
99
100 public enum Severity { CRITICAL, IMPORTANT, TRIVIAL, DOCUMENTATION };
101
102 Severity severity() default Severity.IMPORTANT;
103 String item();
104 String assignedTo();
105 String dateAssigned();
106 }
107
108 /**
109 * Marker annotation to indicate that a method or class
110 * is still in progress.
111 */
112 @Documented
113 @Inherited
114 @Retention(RetentionPolicy.RUNTIME)
115 @interface InProgress { }
116
117 class AnnotationTester {
118
119 @InProgress
120 @GroupTODO(
121 severity=GroupTODO.Severity.CRITICAL,
122 item="Figure out the amount of interest per month",
123 assignedTo="Brett McLaughlin",
124 dateAssigned="04-26-2004"
125 )
126 public void calculateInterest(float amount, float rate) {
127 // Need to finish this method later
128 }
129 }
130