註解##

20.1 基本語法

1. 定義註解

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

標準註解：

元註解：

元註解主要是註解其他的註解。

20.2 編寫註解處理器

public class UserCaseTracer {
	public static void tracerUserCase(List<Integer> userCases,Class<?> c1) {
		for (Method m : c1.getDeclaredMethods()) {
			UseCase uc = m.getAnnotation(UseCase.class);
			if(uc != null){
				  System.out.println("Found Use Case:" + uc.id() +
				          " " + uc.description());
			}
			userCases.remove(new Integer(uc.id()));

		}
		
		 for(int i : userCases) {
		      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);
		    tracerUserCase(useCases, PasswordUtils.class);
	}
}
 

1. 註解元素
   註解元素可用的型別如下：
   所有的基本型別，String，Class，enum，Annotation，以上型別的陣列。

2. 預設值
   註解元素必須有預設值，或者使用必須提供元素值。

3. 生成外部檔案
   快捷方式：如果元素匯中有value時，並且使用註解時該元素是唯一需要賦值的，此時無需用鍵-值對賦值，在括號後給出值即可 @SQLString(30) 。

@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; }
}
 

4. 註解不支援繼承

5. 實現處理器

public class TableCreator {
  public static void main(String[] args) throws Exception {
//    if(args.length < 1) {
//      System.out.println("arguments: annotated classes");
//      System.exit(0);
//    }
    String[] a = {"Member"};
    for(String className : a) {
      Class<?> cl = Class.forName("com.thinkinjava.c20.database.Member");
      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;
  }
}
 

20.3 使用apt處理器註解