问题背景

生产环境上出现空指针异常，追踪报错位置得知以下代码报错

	if (isNull(aiGroup)) {
        return null;
    }
	aiGroup.setNum(isNull(param.getNum()) ? aiGroup.getNum() : param.getNum().doubleValue());

问题排查

乍一看，真没有什么问题（当然可能是我经验不足），细看会发现自动装箱导致空指针异常，上边set方法代码可以拆分为两行：

	Double num = isNull(param.getNum()) ? aiGroup.getNum() : param.getNum().doubleValue();
	aiGroup.setNum(num);

声明：属性类型：aiGroup#num类型是Double，param#num类型是BigDecimal

你可以自己单独写一个main方法，自行运行一下。

在这里我直接说出来，以上代码在获取num时，如果isNull()方法为true时，会从aiGroup获取num，但是aiGroup.getNum()的结果是null，理论是你直接给一个包装类型属性设置null是没有问题的，比如aiGroup.setNum(null)，这样是正确的。

但是三元运算的时候，如果发现结果类型和表达式中的类型不一致，他会在最外层进行自动装箱，会执行Double.valueOf()的操作，所以会出现空指针的现象：Double.valueOf(null)。

反之会出现自动拆箱问题。

排查过程

许多人可能会不明白我是怎么知道为什么会有Double.valueOf(null)这一步，过程很简单，查看Java的字节码就可以看到，想看Java字节码更详细的点可以看：Java字节码介绍。

言归正传，在这里为了简单我又新建了一个简单Main类，使三元运算中表达式的类型不一致，代码如下：


public class Main {
    public static void main(String[] args) {

        SdAiGroup aiGroup = new SdAiGroup();
        // aiGroup#num是Double类型，aiGroup#test是double类型
        Double test = Objects.isNull(aiGroup.getId()) ? aiGroup.getNum() : aiGroup.getTest();
        aiGroup.setAcos(test);
        System.err.println(test);
    }
}

生成class文件，然后执行Javap -c Main.class
结果如下:

Compiled from "Main.java"
public class com.sparkxmedia.xmars.ai.center.service.impl.Main {
  public com.sparkxmedia.xmars.ai.center.service.impl.Main();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: new           #2                  // class com/sparkxmedia/xmars/ai/center/model/entity/SdAiGroup
       3: dup
       4: invokespecial #3                  // Method com/sparkxmedia/xmars/ai/center/model/entity/SdAiGroup."<init>":()V
       7: astore_1
       8: aload_1
       9: invokevirtual #4                  // Method com/sparkxmedia/xmars/ai/center/model/entity/SdAiGroup.getId:()Ljava/lang/Long;
      12: invokestatic  #5                  // Method java/util/Objects.isNull:(Ljava/lang/Object;)Z
      15: ifeq          25
      18: aload_1
      19: invokevirtual #6                  // Method com/sparkxmedia/xmars/ai/center/model/entity/SdAiGroup.getTest:()D
      22: goto          29
      25: aload_1
      26: invokevirtual #6                  // Method com/sparkxmedia/xmars/ai/center/model/entity/SdAiGroup.getTest:()D
      29: invokestatic  #7                  // Method java/lang/Double.valueOf:(D)Ljava/lang/Double;
      32: astore_2
      33: aload_1
      34: aload_2
      35: invokevirtual #8                  // Method com/sparkxmedia/xmars/ai/center/model/entity/SdAiGroup.setAcos:(Ljava/lang/Double;)V
      38: getstatic     #9                  // Field java/lang/System.err:Ljava/io/PrintStream;
      41: aload_2
      42: invokevirtual #10                 // Method java/io/PrintStream.println:(Ljava/lang/Object;)V
      45: return
}

可以看到第29行：Method java/lang/Double.valueOf:(D)Ljava/lang/Double;
将结果执行方法Double.valueOf(null);

反之我们将三元运算中表达式的类型和结果类型一致，代码如下：
将Main方法中三元运算符替换为：Double test = Objects.isNull(aiGroup.getId()) ? aiGroup.getNum() : aiGroup.getNum();
结果如下：

Compiled from "Main.java"
public class com.sparkxmedia.xmars.ai.center.service.impl.Main {
  public com.sparkxmedia.xmars.ai.center.service.impl.Main();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: new           #2                  // class com/sparkxmedia/xmars/ai/center/model/entity/SdAiGroup
       3: dup
       4: invokespecial #3                  // Method com/sparkxmedia/xmars/ai/center/model/entity/SdAiGroup."<init>":()V
       7: astore_1
       8: aload_1
       9: invokevirtual #4                  // Method com/sparkxmedia/xmars/ai/center/model/entity/SdAiGroup.getId:()Ljava/lang/Long;
      12: invokestatic  #5                  // Method java/util/Objects.isNull:(Ljava/lang/Object;)Z
      15: ifeq          25
      18: aload_1
      19: invokevirtual #6                  // Method com/sparkxmedia/xmars/ai/center/model/entity/SdAiGroup.getTest:()D
      22: goto          29
      25: aload_1
      26: invokevirtual #6                  // Method com/sparkxmedia/xmars/ai/center/model/entity/SdAiGroup.getTest:()D
      29: invokestatic  #7                  // Method java/lang/Double.valueOf:(D)Ljava/lang/Double;
      32: astore_2
      33: aload_1
      34: aload_2
      35: invokevirtual #8                  // Method com/sparkxmedia/xmars/ai/center/model/entity/SdAiGroup.setAcos:(Ljava/lang/Double;)V
      38: getstatic     #9                  // Field java/lang/System.err:Ljava/io/PrintStream;
      41: aload_2
      42: invokevirtual #10                 // Method java/io/PrintStream.println:(Ljava/lang/Object;)V
      45: return
}