Support null values when binding properties

[chengaofeng]

Application has a ben annotated with@ConfigurationProperties("test") that has a List<String> attribute called users. at the beginning, one item listed in the external configuration file

test:
  users:
  - Andy

as the application is running, change the configuration file to the following

test:
  users:

or

test:
  users: null

what I expect is users=null or users =[], while the result is still users[0]=Andy

it‘s ok for the following pattern :

test:
  users: ""

Here's a test that fails with Spring Boot 2.1.17.RELEASE

@SpringBootTest(classes=ConfigPropertiesTest.class)
@RunWith(SpringRunner.class)
@EnableConfigurationProperties(ConfigPropertiesTest.TestProperties.class)
@TestPropertySource(properties = {"test.users[0]=Andy"})
public class ConfigPropertiesTest {

    @Autowired
    private TestProperties properties;

    @Autowired
    private ConfigurationPropertiesBindingPostProcessor processor;

    @Autowired
    private ConfigurableEnvironment environment;

    @Test
    public void liveRebindToNull() throws Exception {
        assertEquals(properties.getUsers().get(0),"Andy");
        setNullProperties("test.users");
	assertTrue(environment.getPropertySources().get("test").containsProperty("test.users"));;
        assertNull(environment.getPropertySources().get("test").getProperty("test.users"));
        processor.postProcessBeforeInitialization(properties, "testProperties");
        assertEquals(properties.getUsers().size(), 0); //**_TEST NG_**
    }

    /**
     * TestPropertyValues has no public method set null value, using reflect to do it
     * 
     * @param prefix
     * @throws Exception
     */
    private void setNullProperties(String prefix) throws Exception {
        TestPropertyValues testPropertyValues =  TestPropertyValues.empty();
        Method method = TestPropertyValues.class.getDeclaredMethod("and", Stream.class);
        method.setAccessible(true);

        TestPropertyValues.Pair pair = new TestPropertyValues.Pair(prefix,null);
        testPropertyValues = (TestPropertyValues) method.invoke(testPropertyValues, Stream.of(pair));

        testPropertyValues.applyTo(environment);
    }

    @ConfigurationProperties("test")
    public static class TestProperties {

        private List<String> users;

        public List<String> getUsers() {
            return users;
        }

        public void setUsers(List<String> users) {
            this.users = users;
        }
    }
}

[wilkinsona]

Spring Boot does not support the reloading of properties at runtime. You may be interested in Spring Cloud's support for changing the environment and its refresh scope.

[chengaofeng]

I am sorry not make myself clear.My project is indeed a spring cloud application, The Properties bean is also annotated by @RefreshScope.While The config server is alibaba‘s nacos. When I change the external config file as above(set the value to null), the properties's value didn't change as expect.

In debug mode, when the external config file changed from

test:
  users:
  - Andy

to

test:
  users:

I can find that the propertySource in Environment#propertySources#propertySourceList changed to test.users => null, and then an EnvironmentChangeEvent published. but after the ConfigurationPropertiesBindingPostProcessor.postProcessBeforeInitialization method invoked, the value of the users in the Properties bean is still users[0]=Andy

Then I dig into the source of

org.springframework.boot.context.properties.source.SpringConfigurationPropertySource#find(PropertyMapping mapping)

private ConfigurationProperty find(PropertyMapping mapping) {
	String propertySourceName = mapping.getPropertySourceName();
	Object value = getPropertySource().getProperty(propertySourceName);
	if (value == null) {
		return null;
	}
	...
}	

when the value is null, the method is return null, which means no such item. while in my situation,set test.users=>null is different from not configuring at all

Spring Cloud version： Hoxton.SR8
Spring Boot version： 2.2.5.RELEASE

@wilkinsona,I'm appreciate that you could take a look at this problem again.

[philwebb]

See https://github.com/spring-projects/spring-framework/issues/25142 for background on null values in a MapPropertySource and https://github.com/spring-projects/spring-boot/issues/21542#issuecomment-635107511 for a previous Boot issue.

[philwebb]

This is a limitation of the way that null is handled both in Boot and Spring Framework. I'm afraid the fix for this will be quite involved and is unlikely to happen soon.

[wilkinsona]

#28139 describes another use case for being able to clear the value of a property somehow.

[wilkinsona]

https://github.com/spring-projects/spring-boot/pull/31037 is another case of #28139.

[Sroca3]

@philwebb do you know what all is needed to get this fixed? If https://github.com/spring-projects/spring-framework/issues/19986 is addressed, would it be easy for spring-boot to support nulls? Trying to see how to move this forward.

[wilkinsona]

It's not just https://github.com/spring-projects/spring-framework/issues/19986. Spring Framework's Environment abstraction would also have to be able to distinguish between an absent property and a property with a null value. https://github.com/spring-projects/spring-framework/issues/25142 was opened to explore this but it was effectively rejected and turned into a documentation issue.