config/src/main/java/org/springframework/security/config/annotation/web/messaging/MessageSecurityMetadataSourceRegistry.java
public MessageSecurityMetadataSourceRegistry expressionHandler(SecurityExpressionHandler<Message<Object>> expressionHandler) {
		Assert.notNull(expressionHandler, "expressionHandler cannot be null");
		this.expressionHandler = expressionHandler;
		return this;
	}
public MessageSecurityMetadataSourceRegistry expressionHandler(SecurityExpressionHandler<Message<Object>> expressionHandler) {
		Assert.notNull(expressionHandler, "expressionHandler cannot be null");
		this.expressionHandler = expressionHandler;
		return this;
	}
The {@link SecurityExpressionHandler} to be used. The
default is to use {@link DefaultMessageSecurityExpressionHandler}.

@param expressionHandler the {@link SecurityExpressionHandler} to use. Cannot be null.
@return the {@link MessageSecurityMetadataSourceRegistry} for further
customization.