深入Java注解

注解的基本语法

1.四种元注解的使用说明：

编写注解处理器

1.Java处理注解的过程如图：

代码实例：

//注解接口类@Target(ElementType.METHOD)@Retention(RetentionPolicy.RUNTIME)public @interface UseCase{    public int id();    public String description() default "no description";}//使用注解的客户端类public class PasswordUtils {      @UseCase(id = 47, description =      "Passwords must contain at least one numeric")      public boolean validatePassword(String password) {        return (password.matches("\\w*\\d\\w*"));      }      @UseCase(id = 48)      public String encryptPassword(String password) {       return new StringBuilder(password).reverse().toString();      }      @UseCase(id = 49, description =      "New passwords can't equal previously used ones")      public boolean checkForNewPassword(        List<String> prevPasswords, String password) {        return !prevPasswords.contains(password);      }    }     //注解处理器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);      }    } 

运行结果：
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
2.注解元素：

• 所有基本类型(int,float,boolean等)
• String
• Class
• enum
• Annotation
• 以上类型的数组
  3.在hibernate,enterprise javaBean等这些技术中，常常可以用JPA注解，原理如何实现的呢？这里可以根据上面的流程写一个使用注解创捷数据库。
  代码：

//数据库表名注解接口类@Target(ElementType.TYPE) @Retention(RetentionPolicy.RUNTIME)public @interface DBTable {    //数据库表名  public String name() default "";} //数据库约束注解接口类@Target(ElementType.FIELD)@Retention(RetentionPolicy.RUNTIME)public @interface Constraints {    //主键  boolean primaryKey() default false;  //是否允许为空值  boolean allowNull() default true;  //设置唯一值  boolean unique() default false;}//数据库字符类型的注解类@Target(ElementType.FIELD)@Retention(RetentionPolicy.RUNTIME)public @interface SQLString {  int value() default 0;  String name() default "";  Constraints constraints() default @Constraints;} //数据库整型注解接口类@Target(ElementType.FIELD)@Retention(RetentionPolicy.RUNTIME)public @interface SQLInteger {  String name() default "";  Constraints constraints() default @Constraints;}//使用注解类建立一个数据库//数据库表名@DBTable(name = "MEMBER")public class Member {    //数据库列名  @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; }} //注解处理器public class TableCreator {  public static void main(String[] args) throws Exception {    args=new String[]{"beyondboy.Member"};    for(String className : args) {        //获得类名      Class<?> cl = Class.forName(className);      //获得数据库表名接口类      DBTable dbTable = cl.getAnnotation(DBTable.class);      if(dbTable == null) {        System.out.println(          "No DBTable annotations in class " + className);        continue;      }      //获得数据库表名      String tableName = dbTable.name();      // 如果数据库表名为空，则默认使用类名      if(tableName.length() < 1)        tableName = cl.getName().toUpperCase();      //存储数据库列名      List<String> columnDefs = new ArrayList<String>();      for(Field field : cl.getDeclaredFields()) {        String columnName = null;        Annotation[] anns = field.getDeclaredAnnotations();        //如果字段没有注解，则跳过处理        if(anns.length < 1)          continue;         //判断字段是否是Integer类型        if(anns[0] instanceof SQLInteger) {          SQLInteger sInt = (SQLInteger) anns[0];          //如果列名为空，默认使用字段名          if(sInt.name().length() < 1)            columnName = field.getName().toUpperCase();          else            columnName = sInt.name();          columnDefs.add(columnName + " INT" +            getConstraints(sInt.constraints()));        }        //判断字段是否是String类型        if(anns[0] instanceof SQLString) {          SQLString sString = (SQLString) anns[0];        //如果列名为空，默认使用字段名          if(sString.name().length() < 1)            columnName = field.getName().toUpperCase();          else            columnName = sString.name();          columnDefs.add(columnName + " VARCHAR(" +            sString.value() + ")" +            getConstraints(sString.constraints()));        }        //创建数据据语句        StringBuilder createCommand = new StringBuilder(          "CREATE TABLE " + tableName + "(");        for(String columnDef : columnDefs)          createCommand.append("\n    " + columnDef + ",");        //删除最后一个逗号        String tableCreate = createCommand.substring(          0, createCommand.length() - 1) + ");";        System.out.println("Table Creation SQL for " +          className + " is :\n" + tableCreate);      }    }  }  //获得约束定义  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;  }} 

运行结果：
Table Creation SQL for beyondboy.Member is :
CREATE TABLE MEMBER(
FIRSTNAME VARCHAR(30));
Table Creation SQL for beyondboy.Member is :
CREATE TABLE MEMBER(
FIRSTNAME VARCHAR(30),
LASTNAME VARCHAR(50));
Table Creation SQL for beyondboy.Member is :
CREATE TABLE MEMBER(
FIRSTNAME VARCHAR(30),
LASTNAME VARCHAR(50),
AGE INT);
Table Creation SQL for beyondboy.Member is :
CREATE TABLE MEMBER(
FIRSTNAME VARCHAR(30),
LASTNAME VARCHAR(50),
AGE INT,
HANDLE VARCHAR(30) PRIMARY KEY);
## 3.使用apt处理注解 ##