i = 1 always happens-before v = 2 True. By JLS section 17.4.5, If x and y are actions of the same thread and x comes before y in program order, then hb(x, y). v = 2 happens-before vDst = v in JMM only if it’s actually happens before in time i = 1 happens-before … Read more
The confusion I think you have here is what the author meant by safe publication. He was referring to the safe publication of a non-null Resource, but you seem to get that. Your question is interesting – is it possible to return a null cached value of resource? Yes. The compiler is allowed to reorder … Read more
No, you will never get a NPE. This is because volatile also has the memory-effect of introducing a happens-before relationship. In other words, it will prevent reordering of a = one; b = two; The statements above, will not be re-ordered, and all threads will observe value one for a if b already has value … Read more
The short answer is that JSR-133 goes too far in its explanation. This isn’t a serious issue because JSR-133 is a non-normative document which isn’t part of the language or JVM standards. Rather, it is only a document which explains one possible strategy that is sufficient for implementing the memory model, but isn’t in general … Read more
The idea of a TLAB is to reduce the need of synchronization between threads. Using TLABs, this need is reduced as any thread has an area it can use and expect that it is the only thread using this area. Assuming that a TLAB can hold 100 objects, a thread would only need to aquire … Read more
I haven’t read your question completely, but first of all do not use volatile, even opengles coders do not use it for different ui vs renderer threads. Use volatile if and only if one thread writes (say to some class’ static property) and other reads, even then you have to synchronize, read this for some … Read more
The Java memory model was an important influence on the C++11 memory model, and was where we pulled the terms happens-before and synchronizes-with from. However, the C++11 memory model offers much more fine-grained control over memory ordering than the Java memory model. Java volatile variables are equivalent to C++11 std::atomic<> variables, if you use std::memory_order_acquire … Read more
The main difference between an immutable object (all properties final) and an effectively immutable object (properties aren’t final but can’t be changed) is safe publication. You can safely publish an immutable object in a multi threaded context without having to worry about adding synchronization, thanks to the guarantees provided by the Java Memory Model for … Read more
For HotSpot: The object header consists of a mark word and a klass pointer. The mark word has word size (4 byte on 32 bit architectures, 8 byte on 64 bit architectures) and the klass pointer has word size on 32 bit architectures. On 64 bit architectures the klass pointer either has word size, but … Read more
The FAQ is not the authority on the matter; the JLS is. Section 17.4.4 specifies synchronizes-with relationships, which feed into happens-before relationships (17.4.5). The relevant bullet point is: An unlock action on monitor m synchronizes-with all subsequent lock actions on m (where “subsequent” is defined according to the synchronization order). Since m here is the … Read more