multithreading - Can the synchronized statements be re-ordered by Java compiler for optimization? -


can synchronization statements reordered. i.e : can :

synchronized(a) {    synchronized(b) {      ......    } }

become :

synchronized(b) {      synchronized(a) {       ......       }   }

can synchronization statements reordered?

i assume asking if compiler can reorder synchronized blocks lock order happens in different order code.

the answer no. synchronized block (and volatile field access) impose ordering restrictions on compiler. in case, cannot move monitor-enter before monitor-enter nor monitor-exit after monitor-exit. see grid below.

to quote jsr 133 (java memory model) faq:

it not possible, example, compiler move code before acquire or after release. when acquires , releases act on caches, using shorthand number of possible effects.

doug lea's jsr-133 cookbook has grid shows reordering possibilities. blank entry in grid means reordering allowed. in case, entering synchronized block "monitorenter" (same reordering limitations loading of volatile field) , exiting synchronized block "monitorexit" (same storing volatile field).

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