Threads and Locks. While most of the discussion in. Java Virtual Machine can support many threads. These threads independently execute code that. Threads may be supported by having many hardware processors. The only way for a user to create a thread is to. A thread will start when the start() method. Thread object. This chapter describes the semantics of. As the. specification is similar to the memory models for. Java programming language memory model. When no confusion. Rather, they describe. Any. execution strategy that generates only allowed behaviors is an. The most basic. of these methods is synchronization, which is. Each object in Java. Only one. thread at a time may hold a lock on a monitor. I want to stop my program during 1 second. How to use sleep() method. It is already available in java.lang package. What is difference between sleep method and yield method of. Refer http://preciselyconcise.com/java. The major difference between yield and sleep in Java is that yield() method. Following is the declaration for java.lang.Thread.yield method public static void yield() Parameters NA. Let us compile and run the above program. Any other threads. A thread t may lock a particular monitor. After the lock action has been performed, the body of the. If execution of the body is ever. If the method is an instance method, it locks. If the method is static, it locks the. Class object that represents the class. If execution of the method's body is. Programs where. threads hold (directly or indirectly) locks on multiple objects should. Wait Sets and Notification. Every object, in addition to. A wait set is a set of threads. Elementary actions that add threads to. What are the main uses of yield(), and how does it differ from join() and interrupt()? I am a little bit confused about the use of yield() method in Java. Wait sets are. manipulated solely through the methods. Object. wait. Object. Object. notify. All. Additionally, the. Thread class's methods for sleeping and joining other threads have. One of the following actions occurs. If n is zero (i. e.. Illegal. Monitor. State. Exception is thrown. The thread may be removed from. A notify. action being performed on m in which t is selected. Implementations. are permitted, although not encouraged, to perform. That order does not have to. If the interrupt is deemed to. Interrupted. Exception, and some. If. the notification is deemed to have occurred first, then t. One of the following actions occurs. If n is zero, then. Illegal. Monitor. State. Exception is thrown. This. removal from the wait set enables u's resumption in a wait. Notice, however, that u's lock actions upon resumption. This action causes u's interruption. This enables u to resume in a wait. Interrupted. Exception. The static. method Thread. Interactions of Waits, Notification, and Interruption. The above specifications. Assume that a set s of threads is. Then either. at least one thread in. Interrupted. Exception. Note that if a thread is. Interrupted. Exception, then some other thread in. The thread does not lose ownership of. In particular, the compiler does not have to flush writes. Thread. sleep or Thread. Thread. sleep. or Thread. This would mean that the loop. The Java programming language memory model works by examining each read in. An implementation is free to produce. Incorrectly Synchronized Programs May Exhibit Surprising Behavior. The semantics of the Java programming language allow compilers and. Here are some examples of how incorrectly. This program uses local. A. and B. Initially, A == B ==. Surprising results caused by statement reordering - original. Thread 1. Thread 2. A; 3: r. 1 = B; 2: B = 1; 4: A = 2; It may appear that the result r. Intuitively. either instruction 1 or instruction 3 should come first in an. If instruction 1 comes first, it should not be able to see. If instruction 3 comes first, it should. This is, on the face of it, absurd. If instruction 1 is reordered with. Table 1. 7. 2, then it is easy to see how the. Surprising results caused by statement reordering - valid compiler transformation. Thread 1. Thread 2. B = 1; r. 1 = B; r. A; A = 2; To some programmers, this behavior may seem. However, it should be noted that this code is improperly. When code contains a. A Just- In- Time compiler in a Java Virtual Machine. In addition, the. Java Virtual Machine implementation. In this. chapter, we shall refer to anything that can reorder code as. Initially, p ==. q and p. This program is also. Surprising results caused by forward substitution. Thread 1. Thread 2r. One common compiler optimization involves having the. This situation is shown in. Table 1. 7. 4. Surprising results caused by forward substitution. Thread 1. Thread 2r. Now consider the case where the assignment. Thread 2 happens between the first read. Thread 1. If the compiler decides to reuse the value. From the perspective of the programmer. The actions of. each thread in isolation must behave as governed by the semantics of. When we refer to this, we say that the. To determine if the. If. a is a read, then further evaluation of t uses the value seen by. For conciseness and simplicity in our examples, we often. Most examples consist of two or more threads. We typically use variables. Such variables are not. In this. chapter, we use the term variable to refer to. There are several. Read (normal, or. A volatile read of a variable. A volatile write of a variable. Locking. a monitor. Unlock. Unlocking. An external action is an action that may be. A. thread divergence action is only performed by a thread that is in. If a thread performs a thread divergence. We do not need to concern. As previously. mentioned, all threads need to obey the correct intra- thread semantics. Java programs. We will usually refere to inter- thread actions more. This. may be information as to the success or failure of the action, and any. These parameters are set up by other. They are not explicitly discussed in the. Non- terminating. Programs and Program Order. Among all the inter- thread. Within a sequentially consistent execution. For example, in the trace in. Table 1. 7. 3, as soon as the write. Synchronization Order. Every execution has. A synchronization order. For each thread t, the synchronization order of the. Happens- before Order. Two actions can be ordered. If one. action happens- before another, then the first is. If the reordering produces results. Writes in one. thread that are in a data race with reads in another thread may, for. This. set is unique. Programmers do not need to reason about reorderings to. Therefore they do not. Once the determination that the code is. The use of correct synchronization does not ensure that. However, its use does. Without correct. synchronization, very strange, confusing and counterintuitive. Happens- before Consistency. For the trace in. Table 1. 7. 5, initially A == B ==. The trace can observe r. Behavior allowed by happens- before consistency, but not. Thread 1. Thread 2. B = 1; A = 2; r. 2 = A; r. B; Since there is no synchronization, each read can see. An execution order that displays this behavior is. A; // sees initial write of 0. B; // sees initial write of 0. Another execution order that is happens- before consistent is. A; // sees write of A = 2. B; // sees write of B = 1. In this execution, the reads see writes that occur. This may seem counterintuitive, but is. Allowing. reads to see later writes can sometimes produce unacceptable. Well- Formed Executions. We only consider well- formed. An execution E = < P, A, po, so, W, V, sw. Each read sees a write. For all reads r in. A, we have W(r) in A. W(r). v = r. v. The. It must be a valid partial order. Values. seen by each read are determined by the memory model. The program. order given must reflect the program order in which the actions. P. Executions and Causality Requirements. We use f. For all x in. For. all x,y in d, p(x,y) if and only if. If all. of the actions in A can be committed, then the execution satisfies. Java programming language memory model. All actions in Ci must share the same. Ei. and E. Formally. Ci is a subset of Aihbi. Only the reads. in Ci- 1 need to see the same writes in Ei as in. E. Formally. Vi. Each read r. Ci - Ci- 1 must see writes in Ci- 1 in both Ei and E, but. Ei from the one it sees in. E. Formally. For any read r in. Ai - Ci- 1, we. have hbi(Wi(r). For any read r in. Ci - Ci- 1), we. have Wi(r) in Ci- 1. W(r) in Ci- 1. Given a set of sufficient. Ei, if there is a release- acquire pair. Formally. Let sswi. We. call sswi. the sufficient synchronizes- with edges for. Ei. If sswi(x. y) and hbi(y. Ci. then swj(x, y) for. Happens- before Consistency Is Not Sufficient. Happens- before consistency is a necessary, but not. Merely enforcing happens- before. For. example, happens- before consistency allows values to appear . This can be seen by a detailed examination of the trace in. Table 1. 7. 6. Happens- before consistency is not sufficient. Thread 1. Thread 2r. The code shown in. Table 1. 7. 6 is correctly synchronized. This. may seem surprising, since it does not perform any synchronization. Remember, however, that a program is correctly synchronized. If this code is executed in a sequentially consistent. Since no writes occur, there can be no data races. However, there is an execution of this program that is. This result is happens- before consistent: there is. However, it is clearly not acceptable: there is no. The fact that we allow a read to see a write that comes. As we saw above, the trace in. Table 1. 7. 5 requires some reads to see writes. Since the reads come first in. If that read cannot see a write that occurs later, then it. This is clearly not reflective of all behaviors. In that case, the reads cause. There is. no . Our memory model therefore needs a. It does this by gradually building a set of. Usually, the next action to be committed will reflect the. However, to reflect reads that need to see later writes, we. If, for example, one of the writes in. Table 1. 7. 6 were committed before the read of. Informally, we allow an action to be committed. In Table 1. 7. 6, we cannot. Observable Behavior and Nonterminating Executions. For programs that always. For programs that can fail to terminate in a bounded. A program that, for example, simply prints. If behavior is described by. Programs. can hang if all threads are blocked or if the program can perform an. In such cases, the actions generated by the blocked. Furthermore, if an action y is in O, and. O. Rather. only external actions that are in a set of observable actions are. The. detailed semantics of final fields are somewhat different from those. In particular, compilers have a great deal of.
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