在Spring中是否有任何内置的线程在操作DefaultNamespaceHandlerResolver？

Question

I am debugging an application using Spring. I found something strange When I step into below constructor in DefaultNamespaceHandlerResolver which is called by XmlBeanDefinitionReader. createReaderContext(Resource resource) :

public DefaultNamespaceHandlerResolver(ClassLoader classLoader, String handlerMappingsLocation) {
        Assert.notNull(handlerMappingsLocation, "Handler mappings location must not be null");
        this.classLoader = (classLoader != null ? classLoader : ClassUtils.getDefaultClassLoader());
        this.handlerMappingsLocation = handlerMappingsLocation;
    }

There's a field named handlerMappings in DefaultNamespaceHandlerResolver as below:

private volatile Map<String, Object> handlerMappings;

After I run through above constructor, this field's value will be changed, however,I can't find any clue how its value change, because only classloader and handlerMappingLocation are set in above construtor. I noticed that handlerMappings has been declared as volatile. So my question is ,is there any built-in thread or other thread manipulate this filed in Spring?

Answer 1

Also, this confuses me, I think this is no built-in thread or other thread manipulate this field in Spring.

I find that there is toString() method as follows( here ) which will execute the getHandlerMappings() method.

@Override
public String toString() {
    return "NamespaceHandlerResolver using mappings " + getHandlerMappings();
}

When we debug in IntelliJ or Eclipse, I think the IDE will execute the toString() method in another thread to show the human readable value of the object.

Here is the test code：

public class ToStringTest {

    public static void main(String[] args) {

        WilltoStringInvoked will = new WilltoStringInvoked();

        // #breakpoint1
        System.out.println("If breakpoint here value will be 1");

        /**
         * If we set an breakpoint before this method the output will 
         * be 1, otherwise the output will be 0
         */
        System.out.println(will.getValue());

        // #breakpoint2
        System.out.println("If breakpoint here value will be 0");
    }

    static class WilltoStringInvoked {

        private volatile  int value = 0;

        private int setValue() {
           if (value == 0) {
               synchronized (this) {
                   if (value == 0) {
                       value = 1;
                   }
               }
           }

           return value;
        }

        public int getValue() {
            return value;
        }

        @Override
        public String toString() {
            return "This value is: " + setValue();
        }
    }
}