Java Concurrency in Practice Quotes

“It is far easier to design a class to be thread-safe than to retrofit it for thread safety later.”
― Brian Goetz, Java Concurrency in Practice

“Just as it is a good practice to make all fields private unless they need greater visibility, it is a good practice to make all fields final unless they need to be mutable.”
― Brian Goetz, Java Concurrency in Practice

“Sometimes abstraction and encapsulation are at odds with performance — although not nearly as often as many developers believe — but it is always a good practice first to make your code right, and then make it fast.”
― Brian Goetz, Java Concurrency in Practice

“Locking can guarantee both visibility and atomicity; volatile variables can only guarantee visibility.”
― Brian Goetz, Java Concurrency in Practice

“Immutable objects are simple. They can only be in one state, which is carefully controlled by the constructor. One of the most difficult elements of program design is reasoning about the possible states of complex objects. Reasoning about the state of immutable objects, on the other hand, is trivial.

Immutable objects are also safer. Passing a mutable object to untrusted code, or otherwise publishing it where untrusted code could find it, is dangerous — the untrusted code might modify its state, or, worse, retain a reference to it and modify its state later from another thread. On the other hand, immutable objects cannot be subverted in this manner by malicious or buggy code, so they are safe to share and publish freely without the need to make defensive copies.”
― Brian Goetz, Java Concurrency in Practice

“From the perspective of a class C, an alien method is one whose behavior is not fully specified by C. This includes methods in other classes as well as overrideable methods (neither private nor final) in C itself. Passing an object to an alien method must also be considered publishing that object. Since you can’t know what code will actually be invoked, you don’t know that the alien method won’t publish the object or retain a reference to it that might later be used from another thread.”
― Brian Goetz, Java Concurrency in Practice

“When a field is declared volatile, the compiler and runtime are put on notice that this variable is shared and that operations on it should not be reordered with other memory operations. Volatile variables are not cached in registers or in caches where they are hidden from other processors, so a read of a volatile variable always returns the most recent write by any thread.”
― Brian Goetz, Java Concurrency in Practice

“Whenever more than one thread accesses a given state variable, and one of them might write to it, they all must coordinate their access to it using synchronization.”
― Brian Goetz, Java Concurrency in Practice

“Accessing shared, mutable data requires using synchronization; one way to avoid this requirement is to not share. If data is only accessed from a single thread, no synchronization is needed. This technique, thread confinement, is one of the simplest ways to achieve thread safety. When an object is confined to a thread, such usage is automatically thread-safe even if the confined object itself is not.”
― Brian Goetz, Java Concurrency in Practice

“Once an object escapes, you have to assume that another class or thread may, maliciously or carelessly, misuse it. This is a compelling reason to use encapsulation: it makes it practical to analyze programs for correctness and harder to violate design constraints accidentally.”
― Brian Goetz, Java Concurrency in Practice

“Debugging tip: For server applications, be sure to always specify the -server JVM command line switch when invoking the JVM, even for development and testing. The server JVM performs more optimization than the client JVM, such as hoisting variables out of a loop that are not modified in the loop; code that might appear to work in the development environment (client JVM) can break in the deployment environment (server JVM).”
― Brian Goetz, Java Concurrency in Practice

“Compound actions on shared state, such as incrementing a hit counter (read-modify-write) or lazy initialization (check-then-act), must be made atomic to avoid race conditions. Holding a lock for the entire duration of a compound action can make that compound action atomic. However, just wrapping the compound action with a synchronized block is not sufficient; if synchronization is used to coordinate access to a variable, it is needed everywhere that variable is accessed. Further, when using locks to coordinate access to a variable, the same lock must be used wherever that variable is accessed.”
― Brian Goetz, Java Concurrency in Practice

“The possibility of incorrect results in the presence of unlucky timing is so important in concurrent programming that it has a name: a race condition. A race condition occurs when the correctness of a computation depends on the relative timing or interleaving of multiple threads by the runtime; in other words, when getting the right answer relies on lucky timing.”
― Brian Goetz, Java Concurrency in Practice

“Good uses of volatile variables include ensuring the visibility of their own state, that of the object they refer to, or indicating that an important lifecycle event (such as initialization or shutdown) has occurred.”
― Brian Goetz, Java Concurrency in Practice

“In fact, allocation in Java is now faster than malloc is in C: the common code path for new Object in HotSpot 1.4.x and 5.0 is approximately ten machine instructions.”
― Brian Goetz, Java Concurrency in Practice

“The actual cost of context switching varies across platforms, but a good rule of thumb is that a context switch costs the equivalent of 5,000 to 10,000 clock cycles, or several microseconds on most current processors.”
― Brian Goetz, Java Concurrency in Practice

“Perhaps surprisingly, concurrent programming isn’t so much about threads or locks, any more than civil engineering is about rivets and I-beams. Of course, building bridges that don’t fall down requires the correct use of a lot of rivets and I-beams, just as building concurrent programs require the correct use of threads and locks. But these are just mechanisms—means to an end. Writing thread-safe code is, at its core, about managing access to state, and in particular to shared, mutable state.”
― Brian Goetz, Java Concurrency in Practice

“With the exception of immutable objects, it is not safe to use an object that has been initialized by another thread unless the publication happens-before the consuming thread uses it.”
― Brian Goetz, Java Concurrency in Practice

“Just as tasks should have a cancellation policy, threads should have an interruption policy.”
― Brian Goetz, Java Concurrency in Practice

“An object is immutable if: Its state cannot be modifled after construction; All its flelds are final;[12] and [12] It is technically possible to have an immutable object without all fields being final—String is such a class—but this relies on delicate reasoning about benign data races that requires a deep understanding of the Java Memory Model. (For the curious: String lazily computes the hash code the first time hashCode is called and caches it in a nonfinal field, but this works only because that field can take on only one nondefault value that is the same every time it is computed because it is derived deterministically from immutable state. Don't try this at home.) It is properly constructed (the this reference does not escape during construction).”
― Brian Goetz, Java Concurrency in Practice

“immutability is not equivalent to simply declaring all fields of an object final. An object whose fields are all final may still be mutable, since final fields can hold references to mutable objects.”
― Brian Goetz, Java Concurrency in Practice

“Immutable objects are always thread-safe.”
― Brian Goetz, Java Concurrency in Practice

“An immutable object is one whose state cannot be changed after construction. Immutable objects are inherently thread-safe; their invariants are established by the constructor, and if their state cannot be changed, these invariants always hold.”
― Brian Goetz, Java Concurrency in Practice

“The other end-run around the need to synchronize is to use immutable objects [EJ Item 13]. Nearly all the atomicity and visibility hazards we've described so far, such as seeing stale values, losing updates, or observing an object to be in an inconsistent state, have to do with the vagaries of multiple threads trying to access the same mutable state at the same time. If an object's state cannot be modified, these risks and complexities simply go away. An”
― Brian Goetz, Java Concurrency in Practice

“If you are porting a single-threaded application to a multithreaded environment, you can preserve thread safety by converting shared global variables into ThreadLocals, if the semantics of the shared globals permits this; an applicationwide cache would not be as useful if it were turned into a number of thread-local caches. ThreadLocal is widely used in implementing application frameworks. For example, J2EE containers associate a transaction context with an executing thread for the duration of an EJB call. This is easily implemented using a static Thread-Local holding the transaction context: when”
― Brian Goetz, Java Concurrency in Practice

“you can think of a ThreadLocal as holding a Map that stores the thread-specific values, though this is not how it is actually implemented. The thread-specific values are stored in the Thread object itself; when the thread terminates, the thread-specific values can be garbage collected. If”
― Brian Goetz, Java Concurrency in Practice

“ThreadLocal, which allows you to associate a per-thread value with a value-holding object. Thread-Local provides get and set accessormethods that maintain a separate copy of the value for each thread that uses it, so a get returns the most recent value passed to set from the currently executing thread. Thread-local variables are often used to prevent sharing in designs based on mutable Singletons or global variables. For example, a single-threaded application might maintain a global database connection that is initialized at startup to avoid having to pass a Connection to every method. Since JDBC connections may not be thread-safe, a multithreaded application that uses a global connection without additional coordination is not thread-safe either. By using a ThreadLocal to store the JDBC connection, as in ConnectionHolder in Listing 3.10, each thread will have its own connection. Listing”
― Brian Goetz, Java Concurrency in Practice

“Stack confinement is a special case of thread confinement in which an object can only be reached through local variables. Just as encapsulation can make it easier to preserve invariants, local variables can make it easier to confine objects to a thread. Local variables are intrinsically confined to the executing thread; they exist on the executing thread's stack, which is not accessible to other threads. Stack confinement (also called within-thread or thread-local usage, but not to be confused with the ThreadLocal library class) is simpler to maintain and less fragile than ad-hoc thread confinement. For”
― Brian Goetz, Java Concurrency in Practice

“Because of its fragility, ad-hoc thread confinement should be used sparingly; if possible, use one of the stronger forms of thread confinment (stack confinement or ThreadLocal) instead. 3.3.2.”
― Brian Goetz, Java Concurrency in Practice

“If you are tempted to register an event listener or start a thread from a constructor, you can avoid the improper construction by using a private constructor and a public factory method, as shown in SafeListener in Listing 3.8. Listing”
― Brian Goetz, Java Concurrency in Practice