thinking in java 书中源码 第四版

				//: annotations/database/Constraints.java
				package annotations.database;
				import java.lang.annotation.*;
				
				@Target(ElementType.FIELD)
				@Retention(RetentionPolicy.RUNTIME)
				public @interface Constraints {
				  boolean primaryKey() default false;
				  boolean allowNull() default true;
				  boolean unique() default false;
				} ///:~