What type of loader is used in java

The run-time constant values derived from these structures must thus be values that can be represented using IEEE single and double formats, respectively. The Java Virtual Machine then links the initial class, initializes it, and invokes the public class method void main String[].

The invocation of this method drives all further execution. Execution of the Java Virtual Machine instructions constituting the main method may cause linking and consequently creation of additional classes and interfaces, as well as invocation of additional methods. In an implementation of the Java Virtual Machine, the initial class could be provided as a command line argument. Alternatively, the implementation could provide an initial class that sets up a class loader which in turn loads an application.

Other choices of the initial class are possible so long as they are consistent with the specification given in the previous paragraph. Class or interface creation is triggered by another class or interface D , which references C through its run-time constant pool.

Array classes do not have an external binary representation; they are created by the Java Virtual Machine rather than by a class loader. There are two kinds of class loaders: the bootstrap class loader supplied by the Java Virtual Machine, and user-defined class loaders. Every user-defined class loader is an instance of a subclass of the abstract class ClassLoader.

Applications employ user-defined class loaders in order to extend the manner in which the Java Virtual Machine dynamically loads and thereby creates classes. User-defined class loaders can be used to create classes that originate from user-defined sources. For example, a class could be downloaded across a network, generated on the fly, or extracted from an encrypted file.

A class loader L may create C by defining it directly or by delegating to another class loader. When one class loader delegates to another class loader, the loader that initiates the loading is not necessarily the same loader that completes the loading and defines the class.

If L creates C , either by defining it directly or by delegation, we say that L initiates loading of C or, equivalently, that L is an initiating loader of C. Each such class or interface belongs to a single run-time package. The run-time package of a class or interface is determined by the package name and defining class loader of the class or interface.

If N denotes a nonarray class or an interface, one of the two following methods is used to load and thereby create C :. Otherwise N denotes an array class. However, the defining class loader of D is used in the process of creating array class C.

If an error occurs during class loading, then an instance of a subclass of LinkageError must be thrown at a point in the program that directly or indirectly uses the class or interface being loaded. Given the same name, a good class loader should always return the same Class object. If a class loader L 1 delegates loading of a class C to another loader L 2 , then for any type T that occurs as the direct superclass or a direct superinterface of C , or as the type of a field in C , or as the type of a formal parameter of a method or constructor in C , or as a return type of a method in C , L 1 and L 2 should return the same Class object.

If a user-defined classloader prefetches binary representations of classes and interfaces, or loads a group of related classes together, then it must reflect loading errors only at points in the program where they could have arisen without prefetching or group loading.

We will also represent a class or interface using the notation N L i , where N denotes the name of the class or interface and L i denotes an initiating loader of the class or interface. The following steps are used to load and thereby create the nonarray class or interface C denoted by N using the bootstrap class loader. First, the Java Virtual Machine determines whether the bootstrap class loader has already been recorded as an initiating loader of a class or interface denoted by N.

If so, this class or interface is C , and no class creation is necessary. Otherwise, the Java Virtual Machine passes the argument N to an invocation of a method on the bootstrap class loader to search for a purported representation of C in a platform-dependent manner.

Typically, a class or interface will be represented using a file in a hierarchical file system, and the name of the class or interface will be encoded in the pathname of the file. Note that there is no guarantee that a purported representation found is valid or is a representation of C. This phase of loading must detect the following error:. That class is C. The following steps are used to load and thereby create the nonarray class or interface C denoted by N using a user-defined class loader L.

First, the Java Virtual Machine determines whether L has already been recorded as an initiating loader of a class or interface denoted by N. The value returned by the invocation is the created class or interface C. The remainder of this section describes this process in more detail.

When the loadClass method of the class loader L is invoked with the name N of a class or interface C to be loaded, L must perform one of the following two operations in order to load C :. The class loader L can delegate the loading of C to some other class loader L '.

This is accomplished by passing the argument N directly or indirectly to an invocation of a method on L ' typically the loadClass method. The result of the invocation is C. In either 1 or 2 , if the class loader L is unable to load a class or interface denoted by N for any reason, it must throw an instance of ClassNotFoundException. Since JDK release 1. The argument to loadClass is the name of the class or interface to be loaded.

There is also a two-argument version of the loadClass method, where the second argument is a boolean that indicates whether the class or interface is to be linked or not. Only the two-argument version was supplied in JDK release 1. From JDK release 1. The following steps are used to create the array class C denoted by N using class loader L.

Class loader L may be either the bootstrap class loader or a user-defined class loader. If L has already been recorded as an initiating loader of an array class with the same component type as N , that class is C , and no array class creation is necessary. Otherwise, the following steps are performed to create C :. The Java Virtual Machine creates a new array class with the indicated component type and number of dimensions.

If the component type is a reference type, C is marked as having been defined by the defining class loader of the component type. Otherwise, C is marked as having been defined by the bootstrap class loader. If the component type is a reference type, the accessibility of the array class is determined by the accessibility of its component type. Otherwise, the accessibility of the array class is public. Ensuring type safe linkage in the presence of class loaders requires special care.

It is possible that when two different class loaders initiate loading of a class or interface denoted by N , the name N may denote a different class or interface in each loader. It is essential that any type name N mentioned in the field or method descriptor denote the same class or interface when loaded by L 1 and when loaded by L 2.

After recording that a loader is an initiating loader of a class, the Java Virtual Machine must immediately check to see if any loading constraints are violated. If so, the record is retracted, the Java Virtual Machine throws a LinkageError , and the loading operation that caused the recording to take place fails. If so, the newly imposed loading constraint is retracted, the Java Virtual Machine throws a LinkageError , and the operation that caused the constraint to be imposed either resolution or preparation, as the case may be fails.

The situations described here are the only times at which the Java Virtual Machine checks whether any loading constraints have been violated. A loading constraint is violated if, and only if, all the following four conditions hold:. A full discussion of class loaders and type safety is beyond the scope of this specification. The following steps are used to derive a Class object for the nonarray class or interface C denoted by N using loader L from a purported representation in class file format.

First, the Java Virtual Machine determines whether it has already recorded that L is an initiating loader of a class or interface denoted by N. If so, this creation attempt is invalid and loading throws a LinkageError.

Otherwise, the Java Virtual Machine attempts to parse the purported representation. However, the purported representation may not in fact be a valid representation of C.

This phase of loading must detect the following errors:. UnsupportedClassVersionError , a subclass of ClassFormatError , was introduced to enable easy identification of a ClassFormatError caused by an attempt to load a class whose representation uses an unsupported version of the class file format.

In JDK release 1. Otherwise, if the purported representation does not actually represent a class named N , loading throws an instance of NoClassDefFoundError or an instance of one of its subclasses.

Note that if C is an interface it must have Object as its direct superclass, which must already have been loaded. String class is loaded. It prints the class name, package name, and the names of all available methods of String class. We are using Class. The Class type contains meta-information about a class. For example, type of String. JavaTpoint offers too many high quality services. Mail us on [email protected] , to get more information about given services.

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Data Science. Angular 7. The primordial class loader implements the default implementation of loadClass. Thus, this code understands that the class name java. This code also implements both class path searching and looking into zip files for classes. The really cool thing about the way this is designed is that Java can change its class storage model simply by changing the set of functions that implements the class loader.

Digging around in the guts of the Java virtual machine, you will discover that the primordial class loader is implemented primarily in the functions FindClassFromClass and ResolveClass.

So when are classes loaded? The Java virtual machine has hooks in it to allow a user-defined class loader to be used in place of the primordial one. Furthermore, since the user class loader gets first crack at the class name, the user is able to implement any number of interesting class repositories, not the least of which is HTTP servers -- which got Java off the ground in the first place.

There is a cost, however, because the class loader is so powerful for example, it can replace java. Object with its own version , Java classes like applets are not allowed to instantiate their own loaders. This is enforced by the class loader, by the way. This column will not be useful if you are trying to do this stuff with an applet, only with an application running from the trusted class repository such as local files. A user class loader gets the chance to load a class before the primordial class loader does.

Because of this, it can load the class implementation data from some alternate source, which is how the AppletClassLoader can load classes using the HTTP protocol. A class loader starts by being a subclass of java. The only abstract method that must be implemented is loadClass. The flow of loadClass is as follows:. SimpleClassLoader appears as follows, with descriptions about what it does interspersed with the code.

The code above is the first section of the loadClass method. As you can see, it takes a class name and searches a local hash table that our class loader is maintaining of classes it has already returned. It is important to keep this hash table around since you must return the same class object reference for the same class name every time you are asked for it.

Otherwise the system will believe there are two different classes with the same name and will throw a ClassCastException whenever you assign an object reference between them. It's also important to keep a cache because the loadClass method is called recursively when a class is being resolved, and you will need to return the cached result rather than chase it down for another copy. As you can see in the code above, the next step is to check if the primordial class loader can resolve this class name.

This check is essential to both the sanity and security of the system. Get hold of all the important Java Foundation and Collections concepts with the Fundamentals of Java and Java Collections Course at a student-friendly price and become industry ready. Types of ClassLoaders in Java. Not all classes are loaded by a single ClassLoader.

Depending on the type of class and the path of class, the ClassLoader that loads that particular class is decided. To know the ClassLoader that loads a class the getClassLoader method is used. It is not a java class. Its job is to load the first pure Java ClassLoader.