技術標籤：javase

註解

一、三種java自帶註解

@Override

@Deprecated ------>方法上面加上這個註解說明該方法已棄用。

@SuppressWarnings ------>加上這個註解，idea的警告就看不到了。

二、元註解

@Target 該註解標明瞭自定義註解的使用範圍

@Target(ElementType.TYPE)

//可以選擇的列舉型別
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
}

@Target({ElementType.TYPE,ElementType.FIELD})

@Target內部是一個集合的形式，可以放多個引數。上面程式碼的意思是，該註解既可用於類也可用於屬性。

@Retention 該註解標明瞭自定義註解的生命週期，一般為RUNTIME

@Retention(RetentionPolicy.RUNTIME)

@Inherited 繼承

@Document 是否將註解加入javadoc

前兩個是必須有的註解。

三、自定義註解

@Target({ElementType.TYPE,ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface MyAnnotation {
    int id();
    String name();
}

使用註解時，要麼必須要把定義的屬性都寫上，要麼在定義的屬性的時候後面加上default 數值，否則會報錯。

@MyAnnotation(id=1,name="abc")
public class AnnotationDemo {
    private int a_id;
    private String a_name;
}

設定屬性預設值

@Target({ElementType.TYPE,ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface MyAnnotation {
    int id() default 1;
    String name() default "abc";
}

此時使用@MyAnnotation註解不帶引數也是可以的。

四、反射解析註解

1、類註解

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
public @interface MyRepositoryAnnotation {
    String value();
}

2、屬性註解

@Target({ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface MyColumnAnnotation {
    String column();
    String type();
}

3、在實體類上應用註解

@MyRepositoryAnnotation("MyAccount")
public class Account {
    @MyColumnAnnotation(column = "account_Id",type = "int")
    private int accountId;
    @MyColumnAnnotation(column = "account_Name",type = "String")
    private String accountName;
    @MyColumnAnnotation(column = "account_Owner",type = "String")
    private String accountOwner;
    @MyColumnAnnotation(column = "account_Value",type = "int")
    private int accountValue;

    public Account(int accountId, String accountName, String accountOwner, int accountValue) {
        this.accountId = accountId;
        this.accountName = accountName;
        this.accountOwner = accountOwner;
        this.accountValue = accountValue;
    }

    public Account() {
    }

    public int getAccountId() {
        return accountId;
    }

    public void setAccountId(int accountId) {
        this.accountId = accountId;
    }

    public String getAccountName() {
        return accountName;
    }

    public void setAccountName(String accountName) {
        this.accountName = accountName;
    }

    public String getAccountOwner() {
        return accountOwner;
    }

    public void setAccountOwner(String accountOwner) {
        this.accountOwner = accountOwner;
    }

    public int getAccountValue() {
        return accountValue;
    }

    public void setAccountValue(int accountValue) {
        this.accountValue = accountValue;
    }

    @Override
    public String toString() {
        return "Account{" +
                "accountId=" + accountId +
                ", accountName='" + accountName + '\'' +
                ", accountOwner='" + accountOwner + '\'' +
                ", accountValue='" + accountValue + '\'' +
                '}';
    }
}

4、反射解析註解

public class MyAnnotationReader {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException {
        Class<?> account = Class.forName("com.ouc.a302.bean.Account");
        MyRepositoryAnnotation annotation = account.getAnnotation(MyRepositoryAnnotation.class);
        String value = annotation.value();
        System.out.println("value = " + value);
        Field accountId = account.getDeclaredField("accountId");
        MyColumnAnnotation name = accountId.getAnnotation(MyColumnAnnotation.class);
        String column = name.column();
        System.out.println("column = " + column);
    }
}
ount.getDeclaredField("accountId");
        MyColumnAnnotation name = accountId.getAnnotation(MyColumnAnnotation.class);
        String column = name.column();
        System.out.println("column = " + column);
    }
}