Brian Goetz > 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

“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

“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

“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

“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

“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

“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

“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

“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

“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

“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

“A special case of thread confinement applies to volatile variables. It is safe to perform read-modify-write operations on shared volatile variables as long as you ensure that the volatile variable is only written from a single thread. In this case, you are confining the modification to a single thread to prevent race conditions, and the visibility guarantees for volatile variables ensure that other threads see the most up-to-date value. Because”
― Brian Goetz, Java Concurrency in Practice

“always use the proper synchronization whenever data is shared across threads. 3.1.1.”
― Brian Goetz, Java Concurrency in Practice

“If multiple threads access the same mutable state variable without appropriate synchronization, your program is broken. There are three ways to fix it: Don't share the state variable across threads; Make the state variable immutable; or Use synchronization whenever accessing the state variable.”
― 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

“To ensure thread safety, check-then-act operations (like lazy initialization) and read-modify-write operations (like increment) must always be atomic. We refer collectively to check-then-act and read-modify-write sequences as compound actions: sequences of operations that must be executed atomically in order to remain thread-safe.”
― 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 most common type of race condition is check-then-act, where a potentially stale observation is used to make a decision on what to do next. [4]”
― Brian Goetz, Java Concurrency in Practice

“To preserve state consistency, update related state variables in a single atomic operation.”
― Brian Goetz, Java Concurrency in Practice

“Every shared, mutable variable should be guarded by exactly one lock. Make it clear to maintainers which lock that is.”
― 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

“When designing thread-safe classes, good object-oriented techniques—encapsulation, immutability, and clear specification of invariants—are your best friends.”
― Brian Goetz, Java Concurrency in Practice

“Yet accessing a volatile variable performs no locking and so cannot cause the executing thread to block, making volatile variables a lighter-weight synchronization mechanism than synchronized.[5]”
― Brian Goetz, Java Concurrency in Practice

“There will be times when good object-oriented design techniques are at odds with real-world requirements; it may be necessary in these cases to compromise the rules of good design for the sake of performance or for the sake of backward compatibility with legacy code. 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

“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

“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

“If you haven't considered concurrent access in your class design, some of these approaches can require significant design modifications, so fixing the problem might not be as trivial as this advice makes it sound. It is far easier to design a class to be thread-safe than to retrofit it for thread safety later. In”
― Brian Goetz, Java Concurrency in Practice