《java编程思想》学习笔记20 第20章 注解（annotation）

1，注解为我们在代码中添加信息提供了一种形式化的方法，使我们在以后的的工作中很方便的使用这些数据。

 

2，注解是众多引入java 5中的重要的语言变化之一，java 5 内置了3中注解，在java.lang有注解：

 

  @Override 表示当前的方法定义将覆盖超类的方法。

 

  @Deprecated，如果程序员用了标注为它的元素，那么编译器会发出警告信息。

 

  @SuppressWarrngs，关闭不当的编译器警告。

 

java还另外提供了四种注解，专门负责新注解的创建。

 

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

应用注解：

 

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);
  }
} /* Output:

 

运行的结果：

 

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

 

3，模仿hibernate的注解

 

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

 

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Constraints {
  boolean primaryKey() default false;
  boolean allowNull() default true;
  boolean unique() default false;
} ///:~

 

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

 

@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);
} ///:~

 

处理器  创建表

 

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:

 

创建表的语句是：

 

CREATE TABLE MEMBER(
    FIRSTNAME VARCHAR(30),
    LASTNAME VARCHAR(50),
    AGE INT,
    HANDLE VARCHAR(30) PRIMARY KEY);
很不错。

 

javassist工具类，是jboss的子项目，有关字节码工程的，有空研究一下。