《Spring實戰》第4章--面向切面的Spring--處理通知中的參數（經驗總結）

今天學習《Spring實戰》第4章《面向切面的Spring》，根據4.3.3小節寫出以下切面類：

package proxy;

import java.util.HashMap;
import java.util.Map;
import org.aspectj.lang.ProceedingJoinPoint;
import org.aspectj.lang.annotation.AfterReturning;
import org.aspectj.lang.annotation.Around;
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Pointcut;
import org.springframework.stereotype.Component;

@Component
@Aspect
public class Audience {
    
    private Map<Integer, Integer> trackCounts = new HashMap<>();
    
    @Pointcut("execution(** concert.JayPerform.playTrack(int)) && args(trackNum)")  //<1>
    public void track(int trackNum) {}  //<2>
    
    @AfterReturning("track(trackNum)")  //<3>
    public void countTrack(int trackNum)    //<4>
    {
        
        int currentCount = getPlayCount(trackNum);
        trackCounts.put(trackNum, currentCount+1);
        System.out.println("------- 這首歌播放了"+(currentCount+1)+"次");
    }
    
    public int getPlayCount(int trackNumber)
    {
        return trackCounts.containsKey(trackNumber)?trackCounts.get(trackNumber):0;
    }
}

代碼中 <1>、<2>、<3>、<4> 處，int型參數的名稱都是trackNum，這樣保證了從命名切點到通知方法的參數轉移。而且，這裏的參數trackNum與concert.JayPerform.playTrack(int trackNum) 的參數命名相同。
通過實驗，發現這4處的參數名稱與concert.JayPerform.playTrack(int trackNum)中的參數名稱沒必要相同，只要<1>與<2>處參數名稱相同、<3>與<4>處參數名稱相同便可。這四處代碼可修改以下：

package proxy;

import java.util.HashMap;
import java.util.Map;
import org.aspectj.lang.ProceedingJoinPoint;
import org.aspectj.lang.annotation.AfterReturning;
import org.aspectj.lang.annotation.Around;
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Pointcut;
import org.springframework.stereotype.Component;

@Component
@Aspect
public class Audience {
    
    private Map<Integer, Integer> trackCounts = new HashMap<>();
    
    @Pointcut("execution(** concert.JayPerform.playTrack(int)) && args(num1)")  //<1>
    public void track(int num1) {}  //<2>
    
    @AfterReturning("track(val1)")  //<3>
    public void countTrack(int val1)    //<4>
    {
        
        int currentCount = getPlayCount(val1);
        trackCounts.put(val1, currentCount+1);
        System.out.println("------- 這首歌播放了"+(currentCount+1)+"次");
    }
    
    public int getPlayCount(int trackNumber)
    {
        return trackCounts.containsKey(trackNumber)?trackCounts.get(trackNumber):0;
    }

}