概述
@Retention 指定注释的生存时期
* CLASS:注释记录在类文件中，但在运行时 VM 不需要保留注释（默认）。
* RUNTIME:注释记录在类文件中，在运行时 VM 将保留注释，因此可以使用反射机制读取注释内容。
* SOURCE:编译器要丢弃的注释。
@Target
* 指示注释类型所适用的程序元素的种类，如果注释类型声明中不存在 Target 元注释，
* 则声明的类型可以用在任一程序元素上。
* ElementType.ANNOTATION_TYPE：注释类型声明
* ElementType.CONSTRUCTOR：构造方法声明
* ElementType.FILED：字段声明
* ElementType.LOCAL_VARIABLE：局部变量声明
* ElementType.METHOD：方法声明
* ElementType.PACKAGE：包声明
* ElementType.PARAMETER：参数声明
* ElementType.TYPE：类、借口或枚举声明
● 定义注解
@Target({ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface NotNull {
String message() default "Element is not null";
}
@Target({ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface Null {
String message() default "Element must be null";
}
@Target({ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface Min {
long value() default 0;
}
@Target({ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface Max {
long value();
}
@Target({ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface Size {
int min();
int max();
}
● 实现类（利用反射机制）
public abstract class Validator {
public <T, E> void validate(T t, E e) {
if (e instanceof Field) {
Field f = (Field) e;
validateField(t, f);
} else if (e instanceof Method) {
Method m = (Method) e;
validateMethod(t, m);
}
}
public <T> void validateField(T t, Field f) {
//TODO implements in subclass
}
public <T> void validateMethod(T t, Method m) {
//TODO implements in subclass
}
}
public class NotNullValidator extends Validator {
@Override
public <T> void validateField(T t, Field f) {
if (f.isAnnotationPresent(NotNull.class)) {
try {
f.setAccessible(true);
if (f.get(t) == null) {
throw new ValidatorException(f.getAnnotation(NotNull.class).message());
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
}
public class NullValidator extends Validator {
@Override
public <T> void validateField(T t, Field f) {
if (f.isAnnotationPresent(Null.class)) {
try {
f.setAccessible(true);
if (f.get(t) ！= null) {
throw new ValidatorException(f.getAnnotation(Null.class).message());
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
}
public class MinValidator extends Validator {
@Override
public <T> void validateField(T t, Field f) {
if (f.isAnnotationPresent(Min.class)) {
try {
f.setAccessible(true);
Object value = f.get(t);
if( value instanceof Short || value instanceof Integer || value instanceof Long) {
long val = f.getAnnotation(Min.class).value();
if(Long.valueOf(value.toString()) < val) {
throw new ValidatorException(f.getName()+"'value is not samller than " + val);
}
} else {
throw new ValidatorException(f.getName()+" is not number.");
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
}
public class MaxValidator extends Validator {
@Override
public <T> void validateField(T t, Field f) {
if (f.isAnnotationPresent(Max.class)) {
try {
f.setAccessible(true);
Object value = f.get(t);
if( value instanceof Short || value instanceof Integer || value instanceof Long) {
long val = f.getAnnotation(Max.class).value();
if(Long.valueOf(value.toString()) > val) {
throw new ValidatorException(f.getName()+"'value is not greater than " + val);
}
} else {
throw new ValidatorException(f.getName()+" is not number.");
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
}
public class SizeValidator extends Validator {
@Override
public <T> void validateField(T t, Field f) {
if (f.isAnnotationPresent(Size.class)) {
try {
f.setAccessible(true);
Object value = f.get(t);
if( value instanceof Short || value instanceof Integer || value instanceof Long) {
long max = f.getAnnotation(Size.class).max();
long min = f.getAnnotation(Size.class).min();
long val = Long.valueOf(value.toString());
if(val < min || val > max) {
throw new ValidatorException(f.getName()+"'value is in the " + min + " to " + max + " range");
}
} else {
throw new ValidatorException(f.getName()+" is not number.");
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
}
● 定义验证异常
public class ValidatorException extends Exception{
public ValidatorException() {
super();
}
public ValidatorException(String message) {
super(message);
}
public ValidatorException(String message, Throwable throwable) {
super(message, throwable);
}
public ValidatorException(Throwable throwable) {
super(throwable);
}
}
public enum ValidatorType {
Null,NotNull,Min,Max,Size;
}
public class ValidatorFactory {
public static Validator createValidator(ValidatorType type) {
switch(type) {
case Null : return new NullValidator();
case NotNull : return new NotNullValidator();
case Min : return new MinValidator();
case Max : return new MaxValidator();
case Size : return new SizeValidator();
default : return null;
}
}
}
public class Validation {
public static <T> void validate(T t) {
Field[] fields = t.getClass().getDeclaredFields();
for(Field f : fields) {
Annotation[] as = f.getAnnotations();
for(Annotation a : as) {
String typeName = a.annotationType().getSimpleName();
Validator validator = ValidatorFactory.createValidator(ValidatorType.valueOf(typeName));
validator.validate(t, f);
}
}
}
}
● 测试
public class TestObject {
@NotNull(message="name can not null.")
private String name;
@NotNull(message="age can not null.")
@Max(value = 32)
@Size(max=100, min=10)
private Integer age;
}
public class ClientTest {
public void test() {
TestObject obj = new TestObject();
obj.setName("JOE");
obj.setAge(10);
Validation.validate(obj);
}
}
最后
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