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Honor generic type information when copying properties with BeanUtils

See original GitHub issue

Affects: spring-beans-5.1.2.RELEASE

class XA{}

class XB{}

class A{
    List<XA> a = new ArrayList<>();
    private String b;
    private String c;
    public List<XA> getA() {
        return a;
    }
    public void setA(List<XA> a) {
        this.a = a;
    }

    public String getB() {
        return b;
    }

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

    public String getC() {
        return c;
    }

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

class B {

    List<XB> a = new ArrayList<>();


    private String b;


    private String c;

    public List<XB> getA() {
        return a;
    }

    public void setA(List<XB> a) {
        this.a = a;
    }

    public String getB() {
        return b;
    }

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

    public String getC() {
        return c;
    }

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

public class TestBeanUtils{
        public static void main(String[] args) {
            A a = new A();
            a.getA().add(new XA());
            B b= new B();
            BeanUtils.copyProperties(a,b);
            List<XB> a1 = b.getA();
            for (XB xb : a1) {
                System.out.println("AAA");
            }
            List<XA> a2 = a.getA();
            for (XA xb : a2) {
                System.out.println("AAA");
            }
        }
}

run the main method , then there will throw a java.lang.ClassCastException;

Issue Analytics

State:
Created 4 years ago
Comments:5 (2 by maintainers)

Top GitHub Comments

1reaction

sbrannencommented, Dec 11, 2019

Thank you for raising the issue.

I have distilled the behavior via the following test.

public class BeanUtilsTests {

	@Test
	void test() {
		A a = new A();
		a.getList().add(42);
		B b = new B();

		BeanUtils.copyProperties(a, b);

		assertThat(a.getList()).containsOnly(42);

		b.getList().forEach(n -> assertThat(n).isInstanceOf(Long.class));
		assertThat(b.getList()).isEmpty();
	}
}

class A {

	private List<Integer> list = new ArrayList<>();

	public List<Integer> getList() {
		return list;
	}

	public void setList(List<Integer> list) {
		this.list = list;
	}
}

class B {

	private List<Long> list = new ArrayList<>();

	public List<Long> getList() {
		return list;
	}

	public void setList(List<Long> list) {
		this.list = list;
	}
}

The reason the test fails with a ClassCastException is that the generic type information is ignored in BeanUtils.copyProperties(Object, Object, Class<?>, String...) since it performs a comparison between the return type of the read method in the source and parameter type of the write method in the target, both of which are java.util.List.

Note, however, that the generic type information is available in the java.lang.reflect.Method instances used. Thus, Spring could potentially honor the generic type information when copying properties; however, Spring currently ignores the generic type information.

0reactions

sbrannencommented, Apr 13, 2020

Superseded by #24281