Concurrency Quotes

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

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

It is far easier to design a class to be thread-safe than to retrofit it for thread safety later. ~ Brian Goetz

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

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

Use locking to control access to global variables. Similar to concurrency control in a multiuser database environment, locking requires that before the value of a global variable can be used or updated, the variable must be "checked out." After the variable is used, it's checked back in. During the time it's in use (checked out), if some other part of the program tries to check it out, the lock/unlock routine displays an error message or fires an assertion. ~ Steve McConnell

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

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

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