Java注解--annotation

注解称为元数据，为我们在代码中添加信息提供了一种形式化的方法，使我们可以在稍后摸个时刻非常方便的使用这些数据。——Java编程思想第四版.
注解是众多引入到Java SE5中得语言变化之一。他们可以提供完整的用来描述程序所需的信息，而这些信息是无法使用Java来表达的，注解使得我们可以由编译器来完成测试和验证的格式，存储有关程序的额外信息，属于编译期的类型检查。
定义注解：其中@Target用来定义你的注解将用在山么地方(例如是一个方法还是一个域)。@Retention用来定义该注解在哪一个级别可以使用(源代码中SOURCE，类文件中CLASS或者运行时RUNTIME)。

import java.lang.annitation.*;

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface UseCase{
public int id();
public String description() default "no description";
}

使用注解：

import java.util.*;

public class PasswordUtils{
@UseCase(id = 47, description = "")
public boolean validatePassword(String password){
return password.matches("\w\w");
}

@UseCase(id = 48)
public String encryptPassword(String password){
return new StringBuilder(password.reverse().toString());
}
}

元注解：
@Target(ElementType.[type])

[type]={METHOD, FIELD, TYPE(类、接口、枚举声明), CONSTRUCTOR, LOCAL_VARIABLE, PARAMETER}
@Retention(RetentionPolicy.[policy])
[policy]={SOURCE, CLASS, RUNTIME(反射机制可读取)}
@Documented 表示将此注解包含到Javadoc中

@Inherited 表示允许子类继承父类的注解
标准注解：
注解@Override的代码非常简单，可用于注解类的方法，并在Source级别可用。 并且在Java当中，@Override是可选的
@Desprecated如果程序员使用了注解为他的元素，那么编译器发出警告信息。
@SuppressWarnings 关闭不当的编译器警告信息

[b]注解处理器：有了注解是不够的，还需要注解处理器进行注解的处理，否则注解还没有注释好用

[/b]

import java.lang.reflect.*;
import java.util.*;

public class UseCaseTracker {
public static void trackUseCases(List<Integer> useCases, Class<?> cl) {
for(Method m : cl.getDeclaredMethods()) {
UseCase uc = m.getAnnotation(UseCase.class);
if(uc != null) {
System.out.println("Found Use Case:" + uc.id() +" " + uc.description());
useCases.remove(new Integer(uc.id()));
}
}
for(int i : useCases) {
System.out.println("Warning: Missing use case-" + i);
}
}

public static void main(String[] args) {
List<Integer> useCases = new ArrayList<Integer>();
Collections.addAll(useCases, 47, 48, 49, 50);
trackUseCases(useCases, PasswordUtils.class);
}
}
/* Output:
Found Use Case:47 Passwords must contain at least one numeric
Found Use Case:48 no description
Found Use Case:49 New passwords can't equal previously used ones
Warning: Missing use case-50
*/

这个程序只用了两个反射的接口：getDeclaredMethods()和getAnnotation()，它们都属于AnnotationElement接口(Class、Method和Field
等类都实现了该接口)getAnnotation()返回指定类型注解对象，然后通过调用id()来返回UseCase对象中提取元素的值。

生成外部文件：

import java.lang.annotation.*;

@Target(ElementType.TYPE) // Applies to classes only
@Retention(RetentionPolicy.RUNTIME)
public @interface DBTable {
public String name() default "";
}

@DBTable(name = "MEMBER")
public class Member {
@SQLString(30) String firstName;
@SQLString(50) String lastName;
@SQLInteger Integer age;
@SQLString(value = 30,
constraints = @Constraints(primaryKey = true))
String handle;
static int memberCount;
public String getHandle() { return handle; }
public String getFirstName() { return firstName; }
public String getLastName() { return lastName; }
public String toString() { return handle; }
public Integer getAge() { return age; }
}

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;
}

import java.lang.annotation.*;

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface SQLInteger {
String name() default "";
Constraints constraints() default @Constraints;
}

import java.lang.annotation.*;

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface SQLString {
int value() default 0;
String name() default "";
Constraints constraints() default @Constraints;
}

public @interface Uniqueness {
Constraints constraints()
default @Constraints(unique=true);
}

import java.lang.annotation.*;
import java.lang.reflect.*;
import java.util.*;

public class TableCreator {
public static void main(String[] args) throws Exception {
if(args.length < 1) {
System.out.println("arguments: annotated classes");
System.exit(0);
}
for(String className : args) {
Class<?> cl = Class.forName(className);
DBTable dbTable = cl.getAnnotation(DBTable.class);
if(dbTable == null) {
System.out.println(
"No DBTable annotations in class " + className);
continue;
}
String tableName = dbTable.name();
// If the name is empty, use the Class name:
if(tableName.length() < 1)
tableName = cl.getName().toUpperCase();
List<String> columnDefs = new ArrayList<String>();
for(Field field : cl.getDeclaredFields()) {
String columnName = null;
Annotation[] anns = field.getDeclaredAnnotations();
if(anns.length < 1)
continue; // Not a db table column
if(anns[0] instanceof SQLInteger) {
SQLInteger sInt = (SQLInteger) anns[0];
// Use field name if name not specified
if(sInt.name().length() < 1)
columnName = field.getName().toUpperCase();
else
columnName = sInt.name();
columnDefs.add(columnName + " INT" +
getConstraints(sInt.constraints()));
}
if(anns[0] instanceof SQLString) {
SQLString sString = (SQLString) anns[0];
// Use field name if name not specified.
if(sString.name().length() < 1)
columnName = field.getName().toUpperCase();
else
columnName = sString.name();
columnDefs.add(columnName + " VARCHAR(" +
sString.value() + ")" +
getConstraints(sString.constraints()));
}
StringBuilder createCommand = new StringBuilder(
"CREATE TABLE " + tableName + "(");
for(String columnDef : columnDefs)
createCommand.append("\n    " + columnDef + ",");
// Remove trailing comma
String tableCreate = createCommand.substring(
0, createCommand.length() - 1) + ");";
System.out.println("Table Creation SQL for " +
className + " is :\n" + tableCreate);
}
}
}
private static String getConstraints(Constraints con) {
String constraints = "";
if(!con.allowNull())
constraints += " NOT NULL";
if(con.primaryKey())
constraints += " PRIMARY KEY";
if(con.unique())
constraints += " UNIQUE";
return constraints;
}
} /* Output:
Table Creation SQL for annotations.database.Member is :
CREATE TABLE MEMBER(
FIRSTNAME VARCHAR(30));
Table Creation SQL for annotations.database.Member is :
CREATE TABLE MEMBER(
FIRSTNAME VARCHAR(30),
LASTNAME VARCHAR(50));
Table Creation SQL for annotations.database.Member is :
CREATE TABLE MEMBER(
FIRSTNAME VARCHAR(30),
LASTNAME VARCHAR(50),
AGE INT);
Table Creation SQL for annotations.database.Member is :
CREATE TABLE MEMBER(
FIRSTNAME VARCHAR(30),
LASTNAME VARCHAR(50),
AGE INT,
HANDLE VARCHAR(30) PRIMARY KEY);
*/