Java專題十八：注解 · JavaCook

## Java專題十八：注解 [TOC] ### 18.1. 元注解 | 所在包 | 類名 | 描述 | | --- | --- | --- | | java.lang | `Override` | 表示此方法是重寫的，如果父類或接口中不含該方法，則編譯報錯| | java.lang| `Deprecated` |表示方法已過時，不鼓勵使用，因為方法是危險的或者有其它更好的方法選擇，在程序中使用該方法，會報編譯警告| | java.lang | `SuppressWarnings` |告訴編譯器忽略掉注解中聲明的警告| | java.lang.annotation | `Documented` |表示可以包含在javadoc等工具生成的文檔中| | java.lang.annotation | `Inherited` |表示注解類型會被繼承到子類中| | java.lang.annotation | `Retention` |表示注解類型要保留多久，取值于枚舉類`java.lang.annotation.RetentionPolicy`| | java.lang.annotation | `Target` |表示注解適用那種Java成員，取值于枚舉類`java.lang.annotation.ElementType`| | java.lang.annotation | `Native` |表示一個字段定義常量值引用自native代碼，1.8新增| | java.lang.annotation | `Repeatable` |表示注解可以聲明多次的，1.8新增| 枚舉類`java.lang.annotation.RetentionPolicy`： ~~~ public enum RetentionPolicy { /** * Annotations are to be discarded by the compiler. */ SOURCE, /** * Annotations are to be recorded in the class file by the compiler * but need not be retained by the VM at run time. This is the default * behavior. */ CLASS, /** * Annotations are to be recorded in the class file by the compiler and * retained by the VM at run time, so they may be read reflectively. * * @see java.lang.reflect.AnnotatedElement */ RUNTIME } ~~~ 枚舉類`java.lang.annotation.ElementType`： ~~~ public enum ElementType { /** Class, interface (including annotation type), or enum declaration */ TYPE, /** Field declaration (includes enum constants) */ FIELD, /** Method declaration */ METHOD, /** Formal parameter declaration */ PARAMETER, /** Constructor declaration */ CONSTRUCTOR, /** Local variable declaration */ LOCAL_VARIABLE, /** Annotation type declaration */ ANNOTATION_TYPE, /** Package declaration */ PACKAGE, /** * Type parameter declaration * * @since 1.8 */ TYPE_PARAMETER, /** * Use of a type * * @since 1.8 */ TYPE_USE } ~~~ ### 18.2. 自定義注解 > 假設我們設計一個數據庫Helper類，用于JDBC插入數據，實現類似mybatis的ORM模型首先定義2個注解，注解`Table`標識數據庫中表，適用于`TYPE`類上，注解`Column`標識數據庫中的列，適用于`FIELD`字段上 ~~~ @Documented @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.TYPE) public @interface Table { String value(); } ~~~ ~~~ @Documented @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.FIELD) public @interface Column { String value(); } ~~~ 現在有2個`javabean`類，`Student`類和`Book`類因為數據庫中的字段與我們實體類中字段名字可能不一致，因此我們使用注解標識實體類與數據庫中相關字段的對應關系 ~~~ @Table("student") public class Student { @Column(value = "std_id") int id; @Column(value = "std_name") String name; public int getId() { return id; } public void setId(int id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } } ~~~ ~~~ @Table("book") public class Book { @Column("book_name") String name; @Column("book_author") String author; @Column("price") int price; public String getName() { return name; } public void setName(String name) { this.name = name; } public String getAuthor() { return author; } public void setAuthor(String author) { this.author = author; } public int getPrice() { return price; } public void setPrice(int price) { this.price = price; } } ~~~ 使用反射獲取字段的值，使用注解獲取數據庫中字段的名字，拼接SQL語句 ~~~ import java.lang.reflect.Field; import java.util.logging.Level; import java.util.logging.Logger; public class SQLHelper<T> { final Logger logger = Logger.getLogger(SQLHelper.class.getName()); public int insert(T t){ if (null == t){ throw new NullPointerException(); } StringBuilder builder = new StringBuilder(); Class cls = t.getClass(); Table table = (Table)cls.getAnnotation(Table.class); String tableName = table.value(); Field[] fields = cls.getDeclaredFields(); if (0 == fields.length) { throw new NullPointerException("No Field"); } else if (1 == fields.length) { String columnName = fields[0].getAnnotation(Column.class).value(); Object columnValue = null; try{ columnValue = fields[0].get(t); if (columnValue instanceof String){ columnValue = "\"" + columnValue + "\""; } }catch (IllegalAccessException e){ columnValue = "NULL"; } builder.append("INSERT INTO ").append(tableName) .append(" (").append(columnName).append(") ") .append("VALUES(").append(columnValue).append(");"); } else { StringBuilder columnNames = new StringBuilder(); StringBuilder columnValues = new StringBuilder(); for (int i = 0; i < fields.length; i++) { Column column = fields[i].getAnnotation(Column.class); if (null == column) throw new NullPointerException("Unknown Column"); if (i != fields.length - 1) { columnNames.append(column.value()).append(", "); Object columnValue = null; try{ columnValue = fields[i].get(t); if (columnValue instanceof String){ columnValue = "\"" + columnValue + "\""; } }catch (IllegalAccessException e){ columnValue = "NULL"; } columnValues.append(columnValue).append(", "); } else { columnNames.append(column.value()); Object columnValue = null; try{ columnValue = fields[i].get(t); if (columnValue instanceof String){ columnValue = "\"" + columnValue + "\""; } }catch (IllegalAccessException e){ columnValue = "NULL"; } columnValues.append(columnValue); } } builder.append("INSERT INTO ").append(tableName) .append("(").append(columnNames).append(") ") .append("VALUES(").append(columnValues).append(");"); } return insert0(builder.toString()); } private int insert0(String sql){ logger.log(Level.INFO, sql); //System.out.println(sql); return 1; } } ~~~ 使用如下代碼驗證： ~~~ Book book0 = new Book(); book0.setName("Thinking in java"); book0.setAuthor("Json"); book0.setPrice(80); new SQLHelper<Book>().insert(book0); Student student = new Student(); student.setId(1637); student.setName("Tom"); new SQLHelper<Student>().insert(student); ~~~ 輸出： ``` 四月 14, 2020 1:24:08 上午 SQLHelper insert0 信息: INSERT INTO book(book_name, book_author, price) VALUES("Thinking in java", "Json", 80); 四月 14, 2020 1:24:08 上午 SQLHelper insert0 信息: INSERT INTO student(std_id, std_name) VALUES(1637, "Tom"); ```