Jvm Quotes

The apprentice avoids all use of Java classes. The journeyman embraces Java classes. The master knows which classes to embrace and which to avoid.

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).

UnsafeSequence can be fixed by making getNext a synchronized method, as shown in Sequence in Listing 1.2,[3] thus preventing the unfortunate interaction in Figure 1.1. (Exactly why this works is the subject of Chapters 2 and 3.) [3] @GuardedBy is described in Section 2.4; it documents the synchronization policy for Sequence. Listing 1.2. Thread-safe Sequence Generator. In the absence of synchronization, the compiler, hardware, and runtime are allowed to take substantial liberties with the timing and ordering of actions, such as caching variables in registers or processor-local caches where they are temporarily (or even permanently) invisible to other threads. These tricks are in aid of better performance and are generally desirable, but they place a burden on the developer to clearly identify where data is being shared across threads so that these optimizations do not undermine safety. (Chapter 16 gives the gory details on exactly what ordering guarantees the JVM makes and how synchronization affects those guarantees,

Even if your program never explicitly creates a thread, frameworks may create threads on your behalf, and code called from these threads must be thread-safe. This can place a significant design and implementation burden on developers, since developing thread-safe classes requires more care and analysis than developing non-thread-safe classes. Every Java application uses threads. When the JVM starts, it creates threads for JVM housekeeping tasks (garbage collection, finalization) and a main thread for running the main method. The

The Java Memory Model requires fetch and store operations to be atomic, but for nonvolatile long and double variables, the JVM is permitted to treat a 64-bit read or write as two separate 32-bit operations. If the reads and writes occur in different threads, it is therefore possible to read a nonvolatile long and get back the high 32 bits of one value and the low 32 bits of another.[3] Thus, even if you don't care about stale values, it is not safe to use shared mutable long and double variables in multithreaded programs unless they are declared volatile or guarded by a lock. [3]

Inheriting from Application is shorter than writing an explicit main method, but it also has some shortcomings. First, you can't use this trait if you need to access command-line arguments, because the args array isn't available. For example, because the Summer application uses command-line arguments, it must be written with an explicit main method, as shown in Listing 4.3. Second, because of some restrictions in the JVM threading model, you need an explicit main method if your program is multi-threaded. Finally, some implementations of the JVM do not optimize the initialization code of an object which is executed by the Application trait. So you should inherit from Application only when your program is relatively simple and single-threaded.

That means that a go block compares very favorably to an Elxir process—a very impressive result given that Clojure runs on the JVM, whereas Elixir runs on the Erlang virtual machine, which was built with efficient concurrency in mind.

Originally just for Ruby, Pact now includes JVM and .NET ports

Libraries exist for a number of different platforms that allow our services to send metrics to standard systems. Codahale’s Metrics library is one such example library for the JVM

scripting language is a programming language that provides you with the ability to write scripts that are evaluated (or interpreted) by a runtime environment called a script engine (or an interpreter). A script is a sequence of characters that is written using the syntax of a scripting language and used as the source for a program executed by an interpreter. The interpreter parses the scripts, produces intermediate code, which is an internal representation of the program, and executes the intermediate code. The interpreter stores the variables used in a script in data structures called symbol tables. Typically, unlike in a compiled programming language, the source code (called a script) in a scripting language is not compiled but is interpreted at runtime. However, scripts written in some scripting languages may be compiled into Java bytecode that can be run by the JVM. Java 6 added scripting support to the Java platform that lets a Java application execute scripts written in scripting languages such as Rhino JavaScript, Groovy, Jython, JRuby, Nashorn JavaScript, and so on. Two-way communication is supported. It also lets scripts access Java objects created by the host application. The Java runtime and a scripting language runtime can communicate and make use of each other’s features. Support for scripting languages in Java comes through the Java Scripting API. All classes and interfaces in the Java Scripting API are in the javax.script package. Using a scripting language in a Java application provides several advantages: Most scripting languages are dynamically typed, which makes it simpler to write programs. They provide a quicker way to develop and test small applications. Customization by end users is possible. A scripting language may provide domain-specific features that are not available in Java. Scripting languages have some disadvantages as well. For example, dynamic typing is good to write simpler code; however, it turns into a disadvantage when a type is interpreted incorrectly and you have to spend a lot of time debugging it. Scripting support in Java lets you take advantage of both worlds: it allows you to use the Java programming language for developing statically typed, scalable, and high-performance parts of the application and use a scripting language that fits the domain-specific needs for other parts. I will use the term script engine frequently in this book. A script engine is a software component that executes programs written in a particular scripting language. Typically, but not necessarily, a script engine is an implementation of an interpreter for a scripting language. Interpreters for several scripting languages have been implemented in Java. They expose programming interfaces so a Java program may interact with them.

On the other hand, checkcast meticulously scrutinizes and casts objects, ensuring their harmonious alignment with designated classes.

Hazelcast is an Open Source clustering and highly scalable data distribution plat- form for the JVM.