Answer 1

toolkit's answer is correct. To solve the problem, just declare your Foo's initialise() method to be synchronized. You could refactor Foo as:

private Service service;

public synchronized void initialise() {
    if (service == null) {
        // load external service
        // set field to the loaded service if contacted
    }
}

public String get() {
    if (service == null) {            
        initialise(); // try and load the service again
    }
    // perform operation on the service is service is not null
}

Answered by: Aida304 | Posted: 01-03-2022

Answer 2

Yes, you will have a sync problem.

Lets assume you have single servlet:

public class FooServlet extends HttpServlet {

    private MyBean myBean;

    public void init() {
        myBean = (MyBean) WebApplicationContextUtils.
            getRequiredWebApplicationContext(getServletContext()).getBean("myBean");
    }

    public void doGet(HttpRequest request, HttpResponse response) {
        String string = myBean.get();
        ....
    }

}

class MyBean {
    public String get() {
        if (service == null) {
            // try and load the service again
        }
        // perform operation on the service is service is not null
    }
}

And your bean definition looks like:

<bean id="myBean" class="com.foo.MyBean" init-method="initialise" />

The problem is that your servlet instance is used by multiple request threads. Hence, the code block guarded by service == null may be entered by multiple threads.

The best fix (avoiding double-checked-locking etc) is:

class MyBean {
    public synchronized String get() {
        if (service == null) {
            // try and load the service again
        }
        // perform operation on the service is service is not null
    }
}

Hope this makes sense. Drop a comment if not.

Answered by: John517 | Posted: 01-03-2022

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