Java反射报错“Attempt to get int field “i“ with illegal data type conversion to short“

Posted 2021-09-03 二木成林

异常

异常的堆栈跟踪信息如下：

Exception in thread "main" java.lang.IllegalArgumentException: Attempt to get int field "com.demo.bean.demo.Bird.i" with illegal data type conversion to short
	at sun.reflect.UnsafeFieldAccessorImpl.newGetIllegalArgumentException(UnsafeFieldAccessorImpl.java:69)
	at sun.reflect.UnsafeFieldAccessorImpl.newGetShortIllegalArgumentException(UnsafeFieldAccessorImpl.java:128)
	at sun.reflect.UnsafeIntegerFieldAccessorImpl.getShort(UnsafeIntegerFieldAccessorImpl.java:52)
	at java.lang.reflect.Field.getShort(Field.java:537)
	at com.demo.bean.demo.Test.main(Test.java:21)

错误代码

public class Test {
    public static void main(String[] args) throws IOException, IllegalAccessException, NoSuchFieldException {
        Bird bird = new Bird((byte) 1, 2, (short) 3, 4L, 5.0F, 6.0, false, 'X', "text");
        Class<? extends Bird> birdClass = bird.getClass();
        Field[] fields = birdClass.getFields();

        for (Field field : fields) {
            short s = field.getShort(bird);
            System.out.println(s);
        }
    }
}

class Animal {
    public String name;
}

class Bird extends Animal {
    public byte b;
    public int i;
    public short s;
    public long l;
    public float f;
    public double d;
    public boolean bool;
    public char c;
    public String str;

    public Bird() {
    }

    public Bird(byte b, int i, short s, long l, float f, double d, boolean bool, char c, String str) {
        this.b = b;
        this.i = i;
        this.s = s;
        this.l = l;
        this.f = f;
        this.d = d;
        this.bool = bool;
        this.c = c;
        this.str = str;
    }

    public byte getB() {
        return b;
    }

    public void setB(byte b) {
        this.b = b;
    }

    public int getI() {
        return i;
    }

    public void setI(int i) {
        this.i = i;
    }

    public short getS() {
        return s;
    }

    public void setS(short s) {
        this.s = s;
    }

    public long getL() {
        return l;
    }

    public void setL(long l) {
        this.l = l;
    }

    public float getF() {
        return f;
    }

    public void setF(float f) {
        this.f = f;
    }

    public double getD() {
        return d;
    }

    public void setD(double d) {
        this.d = d;
    }

    public boolean isBool() {
        return bool;
    }

    public void setBool(boolean bool) {
        this.bool = bool;
    }

    public char getC() {
        return c;
    }

    public void setC(char c) {
        this.c = c;
    }

    public String getStr() {
        return str;
    }

    public void setStr(String str) {
        this.str = str;
    }
}

原因

从打印的堆栈跟踪信息中，我们可以得知试图去获取Bird类中的i字段的值，我们去查看Bird类中的成员变量i发现它的数据类型是short，但我们在遍历Field[]数组时，不是所有字段的成员变量数据类型都是short，比如Bird类中的成员变量b是byte类型，成员变量i是int类型，而第三个成员变量s的数据类型才是short，在short前面的成员变量或后面的成员变量是无法通过getShort()方法获取到值的，所以会抛出违法参数的异常。

解决

对Field字段所表示的成员变量的数据类型进行判断，如果Field所表示的成员变量是short类型的，就采用getShort()方法，如果是int类型的，则采用getInt()方法。也就是只需要判断八种基本数据类型而已，但如果不想判断，那么可以使用get()方法，返回的是一个Object结果。

正确代码

public class Test {
    public static void main(String[] args) throws IOException, IllegalAccessException, NoSuchFieldException {
        Bird bird = new Bird((byte) 1, 2, (short) 3, 4L, 5.0F, 6.0, false, 'X', "text");
        Class<? extends Bird> birdClass = bird.getClass();
        Field[] fields = birdClass.getFields();

        for (Field field : fields) {
            if ("short".equals(field.getGenericType().getTypeName())) {
                short s = field.getShort(bird);
                System.out.println(s);
            }
        }
    }
}