Class loader in java

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This class loader is used to load classes and resources from a search path of URLs referring to both JAR files and directories. Any jar: scheme URL (see JarURLConnection) is assumed to refer to a JAR file. Any file: scheme URL that ends with a '/' is assumed to refer to a directory. Otherwise, the URL is assumed to refer to a JAR file which will be opened as needed. This class loader supports the loading of classes and resources from the contents of a The AccessControlContext of the thread that created the instance of URLClassLoader will be used when subsequently loading classes and resources. The classes that are loaded are by default granted permission only to access the URLs specified when the URLClassLoader was created. Since: 1.2 Constructs a new URLClassLoader for the given URLs. The URLs will be searched in the order specified for classes and resources after first searching in the specified parent class loader. Any jar: scheme URL is assumed to refer to a JAR file. Any file: scheme URL that ends with a '/' is assumed to refer to a directory. Otherwise, the URL is assumed to refer to a JAR file which will be downloaded and opened as needed. If there is a security manager, this method first calls the security manager's checkCreateClassLoader method to ensure creation of a class loader is allowed. Parameters: urls - the URLs from which to load classes and resources parent - the parent class loader for delegation Throws: checkCreateClassLoader method doesn't allow creat...

While working in Java, we often use a large number of classes. These Java classes are not loaded all at once in the memory, instead, they are loaded when required by an application. This is where Java ClassLoaders come into the picture. So in this article, I will discuss how to use the ClassLoader in Java along with examples. The following topics will be covered in this article: • • • • • Let us get started! What is ClassLoader in Java? ClassLoader in Java is called by the Java Runtime Environment to dynamically load the classes whenever required by the application in the Now, let us understand the different types of built-in ClassLoaders in Java. Types of ClassLoader in Java The different types of ClassLoaders in Java are as follows: • • • Let us discuss each one of them one by one. Extension ClassLoader As the name suggests the Extension ClassLoader loads the extensions of the core Java classes from the Application or System ClassLoader The Application or the System ClassLoader is a child of the Extension ClassLoader. This type of ClassLoader loads all the application level classes found in the -cp command-line option or int the CLASSPATH environment variable. Bootstrap ClassLoader As we all know that java.lang.ClassLoade. But, since ClassLoaders are classes, the Bootstrap ClassLoader is responsible to load the JDK internal classes. BootStrap ClassLoader is a machine code that starts the operation when JVM calls it and loads the classes from rt.jar. So, you can understan...

If you're using a Sun/Oracle JVM, you could use the TraceClassLoading and TraceClassUnloading options. Use the following to see what options your JVM supports: java -XX:+AggressiveOpts -XX:+UnlockDiagnosticVMOptions -XX:+UnlockExperimentalVMOptions -XX:+PrintFlagsFinal -version If these options are supported, run your Java application using -XX:+TraceClassLoading -XX:+TraceClassUnloading. You should see messages like: [Loaded ... from ...] [Unloading class ...] From Java 9 onward, you can use -Xlog. e.g.: java -Xlog:class+load -Xlog:class+unload ... This will print entries like: [0.296s][info][class,load] java.lang.Shutdown source: jrt:/java.base [0.296s][info][class,load] java.lang.Shutdown$Lock source: jrt:/java.base You can also use -Xlog:help to get more information on the option. You can use static block to detect class loading but you can not detect class unloading. In java all classes loaded through the system classloader will never be unloaded and all classes loaded through other classloaders will be unloaded only when the classloader is unloaded. static

A class loader is an object that is responsible for loading classes. The class ClassLoader is an abstract class. Given the Every Class object contains a reference to the ClassLoader that defined it. Class objects for array classes are not created by class loaders, but are created automatically as required by the Java runtime. The class loader for an array class, as returned by Class.getClassLoader() is the same as the class loader for its element type; if the element type is a primitive type, then the array class has no class loader. Applications implement subclasses of ClassLoader in order to extend the manner in which the Java virtual machine dynamically loads classes. Class loaders may typically be used by security managers to indicate security domains. The ClassLoader class uses a delegation model to search for classes and resources. Each instance of ClassLoader has an associated parent class loader. When requested to find a class or resource, a ClassLoader instance will delegate the search for the class or resource to its parent class loader before attempting to find the class or resource itself. The virtual machine's built-in class loader, called the "bootstrap class loader", does not itself have a parent but may serve as the parent of a ClassLoader instance. Class loaders that support concurrent loading of classes are known as parallel capable class loaders and are required to register themselves at their class initialization time by invoking the ClassLoader.registe...

Now figuring out the connection between class loading and static block after having an idea over the static block and class loading, it is found that execution of a static block happens when a class gets loaded for the first time. It is a series of steps. Illustration: Showcasing generic execution of that static block is supposed to happen with series of steps as mentioned. Randomly considering a java file ‘File.java’, having a static block in it is followed by a series of steps as mentioned. • Compilation of java file. • Execution of java file. • Java virtual machine JVM is calling main method in the program. • Class is loaded and all the necessary information is stored in memory by now. • Execution of static block begins. Example

The Java Class Loader is a part of the A Each Java class must be loaded by a class loader. When the JVM is started, three class loaders are used: • Bootstrap class loader • Extensions class loader • System class loader The bootstrap class loader loads the core Java libraries /jre/lib (or /jmods> for Java 9 and above) directory. This class loader, which is part of the core JVM, is written in native code. The extensions class loader loads the code in the extensions directories ( /jre/lib/ext, java.ext.dirs system property). The system class loader loads code found on java.class.path, which maps to the User-defined class loaders [ ] The Java class loader is written in Java. It is therefore possible to create your own class loader without understanding the finer details of the Java Virtual Machine. Every Java class loader has a parent class loader, defined when a new class loader is instantiated or set to the virtual machine's system default class loader. This makes it possible (for example): • to load or unload classes at runtime (for example to load libraries dynamically at runtime, even from an • implementing scripting languages, such as • using • allowing user-defined • allowing multiple • to change the way the • to modify the loaded bytecode (for example, for load-time Class Loaders in Jakarta EE [ ] JAR hell [ ] JAR hell is a term similar to • Accidental presence of two different versions of a library installed on a system. This will not be considered an error by the sys...