The org.springframework.boot.context.properties.bind.MapBinder#merge works in wrong result by deleting the cofniguration

the code like this:

@Data
@ToString
@ConfigurationProperties(value = MonitorPluginConfiguration.PREFIX)
public class MonitorPluginConfiguration {
    public static final String PREFIX = "monitor";
    private Map<String, String> plugins;
}

the yaml configuration in nacos:

monitor:
  plugins:
    p1: 1
    p2: 2
    p3: 3
    p4: 4

if i delete the key-value p4: 4 in the configuration from nacos ，the value for field MonitorPluginConfiguration#plugins will not be deleted.

the changed configuration in nacos after delete the the key-value p4: 4

monitor:
  plugins:
    p1: 1
    p2: 2
    p3: 3

I trace the code , i find the method in org.springframework.boot.context.properties.bind.MapBinder#merge，

        @Override
	protected Map<Object, Object> merge(Supplier<Map<Object, Object>> existing, Map<Object, Object> additional) {
		Map<Object, Object> existingMap = getExistingIfPossible(existing);
		if (existingMap == null) {
			return additional;
		}
		try {
			existingMap.putAll(additional);
			return copyIfPossible(existingMap);
		}
		catch (UnsupportedOperationException ex) {
			Map<Object, Object> result = createNewMap(additional.getClass(), existingMap);
			result.putAll(additional);
			return result;
		}
	}

the parmater additional value is {p1=1, p2=2, p3=3}

and the variable existingMap value is {p1=1, p2=2, p3=3, p4=4}

for the code existingMap.putAll(additional), it will save the original value {p1=1, p2=2, p3=3, p4=4}. so the result is bad. I think that it is good to execute existingMap.clear() before executing the code existingMap.putAll(additional) The code like this:

        @Override
	protected Map<Object, Object> merge(Supplier<Map<Object, Object>> existing, Map<Object, Object> additional) {
		Map<Object, Object> existingMap = getExistingIfPossible(existing);
		if (existingMap == null) {
			return additional;
		}
		try {
			existingMap.clear(); // add this line
			existingMap.putAll(additional);
			return copyIfPossible(existingMap);
		}
		catch (UnsupportedOperationException ex) {
			Map<Object, Object> result = createNewMap(additional.getClass(), existingMap);
			result.putAll(additional);
			return result;
		}
	}

[wilkinsona]

Thanks. From a Spring Boot perspective, this is working as designed and documented.

I believe the bug is in Spring Cloud. When they are rebinding configuration properties, an existing instance of MonitorPluginConfiguration should not be re-used. Instead, a fresh instance should be created and then have properties bound to it. This has been raised before but the Spring Cloud team declined it as a duplicate of https://github.com/spring-cloud/spring-cloud-commons/issues/818. You may want to raise the problem again as, IMO, it should be documented as a limitation at the least.

[wilkinsona]

@winjaychan Have you read the reference documentation that I linked to earlier? It describes the difference in behaviour for binding lists and maps. Lists are only ever bound from a single source (so they are cleared). Maps can be bound from multiple sources (so they are not cleared).

If you have any further questions, please follow up on Stack Overflow or Gitter. As mentioned in the guidelines for contributing, we prefer to use GitHub issues only for bugs and enhancements.