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};
Thread thdB = new Thread(r2);
thdA.start();
thdB.start();
}
}
stanceMethod2()
, respectively, at different times. Consider the following execu-
tion sequence:
1. Thread A calls
instanceMethod1()
, obtains the lock assigned to the
lock1
-referenced object, and enters its outer critical section (but has not yet
acquired the lock assigned to the
lock2
-referenced object).
2. Thread B calls
instanceMethod2()
, obtains the lock assigned to the
lock2
-referenced object, and enters its outer critical section (but has not yet
acquired the lock assigned to the
lock1
-referenced object).
3. ThreadAattemptstoacquirethelockassociatedwith
lock2
.TheJVMforces
the thread to wait outside of the inner critical section because thread B holds
that lock.
4. ThreadBattemptstoacquirethelockassociatedwith
lock1
.TheJVMforces
the thread to wait outside of the inner critical section because thread A holds
that lock.
5. Neitherthreadcanproceedbecausetheotherthreadholdstheneededlock.We
have a deadlock situation and the program (at least in the context of the two
threads) freezes up.
Althoughthepreviousexampleclearlyidentifiesadeadlockstate,itisoftennotthat
easy to detect deadlock. For example, your code might contain the following circular
relationship among various classes (in several source files):
• Class A's synchronized method calls class B's synchronized method.
• Class B's synchronized method calls class C's synchronized method.
• Class C's synchronized method calls class A's synchronized method.
IfthreadAcallsclassA'ssynchronizedmethodandthreadBcallsclassC'ssynchron-
izedmethod,threadBwillblockwhenitattemptstocallclassA'ssynchronizedmethod
and thread A is still inside of that method. Thread A will continue to execute until it
calls class C's synchronized method, and then block. Deadlock results.
