Locking can guarantee both visibility and atomicity; volatile variables can only guarantee visibility.
Brian GoetzStichwörter: programming java concurrency
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 GoetzStichwörter: programming java concurrency
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 GoetzStichwörter: programming java concurrency
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 GoetzStichwörter: programming java concurrency
It is far easier to design a class to be thread-safe than to retrofit it for thread safety later.
Brian GoetzStichwörter: programming java concurrency
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 GoetzStichwörter: programming java concurrency
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 GoetzStichwörter: programming java concurrency
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 GoetzStichwörter: programming java concurrency
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 GoetzStichwörter: programming java concurrency
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 GoetzStichwörter: programming java concurrency
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