Databases
StackOverflow2013
Postsdbo
POWhat is the memory visibility of variables accessed in static singletons in Java?
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POWhat is the memory visibility of variables accessed in static singletons in Java?
    primarykey
    data
    text
    <p>I've seen this type of code a lot in projects, where the application wants a global data holder, so they use a static singleton that any thread can access.</p> <pre><code>public class GlobalData { // Data-related code. This could be anything; I've used a simple String. // private String someData; public String getData() { return someData; } public void setData(String data) { someData = data; } // Singleton code // private static GlobalData INSTANCE; private GlobalData() {} public synchronized GlobalData getInstance() { if (INSTANCE == null) INSTANCE = new GlobalData(); return INSTANCE; } } </code></pre> <p>I hope it's easy to see what's going on. One can call <code>GlobalData.getInstance().getData()</code> at any time on any thread. If two threads call setData() with different values, even if you can't guarantee which one "wins", I'm not worried about that.</p> <p>But thread-safety isn't my concern here. What I'm worried about is memory visibility. Whenever there's a memory barrier in Java, the cached memory is synched between the corresponding threads. A memory barrier happens when passing through synchronizations, accessing volatile variables, etc.</p> <p>Imagine the following scenario happening in chronological order:</p> <pre><code>// Thread 1 GlobalData d = GlobalData.getInstance(); d.setData("one"); // Thread 2 GlobalData d = GlobalData.getInstance(); d.setData("two"); // Thread 1 String value = d.getData(); </code></pre> <p>Isn't it possible that the last value of <code>value</code> in thread 1 can still be <code>"one"</code>? The reason being, thread 2 never called any synchronized methods after calling <code>d.setData("two")</code> so there was never a memory barrier? Note that the memory-barrier in this case happens every time <code>getInstance()</code> is called because it's synchronized.</p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USMatt Quigley
    1. USMatt Quigley
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    3. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POWhat is the memory visibility of variables accessed in static singletons in Java?
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. POWhat is the memory visibility of variables accessed in static singletons in Java?
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. POWhat is the memory visibility of variables accessed in static singletons in Java?
      1. This table or related slice is empty.
      1. VTUpMod
    1. COYou say `One can call GlobalData.getInstance().getData() at any time on any thread. It's thread-safe so I'm not worried about that` but it's actually not thread-safe exactly because of the scenario that you describe. Thread safety is not only about seeing an object in an inconsistent state. It's also about seeing stale data.
      singulars
      1. POWhat is the memory visibility of variables accessed in static singletons in Java?
      1. USyair
    2. CO@yair Yes but I was really trying to examine the memory-visibility aspect as opposed to inconsistent states based on synchronous execution.
      singulars
      1. POWhat is the memory visibility of variables accessed in static singletons in Java?
      1. USMatt Quigley
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload