Hotspot Garbage Collection

Thought of the Day: The production version of Hotspot has 211 command line options for garbage collection. Getting the right combination of options is hard.

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Double Checked Locking

I still get a lot of questions about whether double-checked locking works in Java, and I should probably post something to clear it up. And I'll plug Josh Bloch's new book, too. Double Checked Locking is this idiom: // Broken -- Do Not Use! class Foo { private Helper helper = null; public Helper getHelper() { if (helper == null) { synchronized(this) { if (helper == null) { helper = new Helper(); } } } return helper; } The point of this code is to avoid synchronization when the object has already been constructed. This code doesn't work in Java. The basic principle is that compiler transformations (this includes the JIT, which is the optimizer that the JVM uses...

What Volatile Means in Java

Today, I'm going to talk about what volatile means in Java. I've sort-of covered this in other posts, such as my posting on the ++ operator , my post on double-checked locking and the like, but I've never really addressed it directly. First, you have to understand a little something about the Java memory model. I've struggled a bit over the years to explain it briefly and well. As of today, the best way I can think of to describe it is if you imagine it this way: Each thread in Java takes place in a separate memory space (this is clearly untrue, so bear with me on this one). You need to use special mechanisms to guarantee that communication happens between these threads, as you would on a message passing system. Memory writes that happen in one thread can "leak through" and be seen by another thread, but this is by no means guaranteed. Without explicit communication, you can't guarantee which writes get seen by other threads, or even the order in whic...

Atomicity, Visibility and Ordering

(Note: I've cribbed this from my doctoral dissertation. I tried to edit it heavily to ease up on the mangled academic syntax required by thesis committees, but I may have missed some / badly edited in places. Let me know if there is something confusingly written or just plain confusing, and I'll try to untangle it.) There are these three concepts, you see. And they are fundamental to correct concurrent programming. When a concurrent program is not correctly written, the errors tend to fall into one of the three categories: atomicity , visibility , or ordering . Atomicity deals with which actions and sets of actions have indivisible effects. This is the aspect of concurrency most familiar to programmers: it is usually thought of in terms of mutual exclusion. Visibility determines when the effects of one thread can be seen by another. Ordering determines when actions in one thread can be seen to occur out of order with respect to another. Let's talk about t...

Java Concurrency (&c)

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