RMIClassLoader (Java Platform SE 8 )
Keywords: java.rmi.server.RMIClassLoader class
Date: 2016-04-24

compact2, compact3
java.rmi.server

## Class RMIClassLoader

- java.lang.Object
- - java.rmi.server.RMIClassLoader

-     public class RMIClassLoader
      extends Object

  `RMIClassLoader` comprises static methods to support dynamic class loading with RMI. Included are methods for loading classes from a network location (one or more URLs) and obtaining the location from which an existing class should be loaded by remote parties. These methods are used by the RMI runtime when marshalling and unmarshalling classes contained in the arguments and return values of remote method calls, and they also may be invoked directly by applications in order to mimic RMI's dynamic class loading behavior.

  The implementation of the following static methods

  - `loadClass(URL,String)`
  - `loadClass(String,String)`
  - `loadClass(String,String,ClassLoader)`
  - `loadProxyClass(String,String[],ClassLoader)`
  - `getClassLoader(String)`
  - `getClassAnnotation(Class)`
  is provided by an instance of `RMIClassLoaderSpi`, the service provider interface for those methods. When one of the methods is invoked, its behavior is to delegate to a corresponding method on the service provider instance. The details of how each method delegates to the provider instance is described in the documentation for each particular method.

  The service provider instance is chosen as follows:

  - If the system property `java.rmi.server.RMIClassLoaderSpi` is defined, then if its value equals the string `"default"`, the provider instance will be the value returned by an invocation of the `getDefaultProviderInstance()` method, and for any other value, if a class named with the value of the property can be loaded by the system class loader (see `ClassLoader.getSystemClassLoader()`) and that class is assignable to `RMIClassLoaderSpi` and has a public no-argument constructor, then that constructor will be invoked to create the provider instance. If the property is defined but any other of those conditions are not true, then an unspecified `Error` will be thrown to code that attempts to use `RMIClassLoader`, indicating the failure to obtain a provider instance.
  - If a resource named `META-INF/services/java.rmi.server.RMIClassLoaderSpi` is visible to the system class loader, then the contents of that resource are interpreted as a provider-configuration file, and the first class name specified in that file is used as the provider class name. If a class with that name can be loaded by the system class loader and that class is assignable to `RMIClassLoaderSpi` and has a public no-argument constructor, then that constructor will be invoked to create the provider instance. If the resource is found but a provider cannot be instantiated as described, then an unspecified `Error` will be thrown to code that attempts to use `RMIClassLoader`, indicating the failure to obtain a provider instance.
  - Otherwise, the provider instance will be the value returned by an invocation of the `getDefaultProviderInstance()` method.
  Since:
    JDK1.1
  See Also:
    `RMIClassLoaderSpi`

- - ### Method Summary

    All Methods Static Methods Concrete Methods Deprecated Methods
    Modifier and Type | Method and Description
    --- | ---
    `static String` | `getClassAnnotation(Class<?> cl)` Returns the annotation string (representing a location for the class definition) that RMI will use to annotate the class descriptor when marshalling objects of the given class.
    `static ClassLoader` | `getClassLoader(String codebase)` Returns a class loader that loads classes from the given codebase URL path.
    `static RMIClassLoaderSpi` | `getDefaultProviderInstance()` Returns the canonical instance of the default provider for the service provider interface `RMIClassLoaderSpi`.
    `static Object` | `getSecurityContext(ClassLoader loader)` Deprecated. no replacement. As of the Java 2 platform v1.2, RMI no longer uses this method to obtain a class loader's security context.
    `static Class<?>` | `loadClass(String name)` Deprecated. replaced by `loadClass(String,String)` method
    `static Class<?>` | `loadClass(String codebase, String name)` Loads a class from a codebase URL path.
    `static Class<?>` | `loadClass(String codebase, String name, ClassLoader defaultLoader)` Loads a class from a codebase URL path, optionally using the supplied loader.
    `static Class<?>` | `loadClass(URL codebase, String name)` Loads a class from a codebase URL.
    `static Class<?>` | `loadProxyClass(String codebase, String[] interfaces, ClassLoader defaultLoader)` Loads a dynamic proxy class (see `Proxy`) that implements a set of interfaces with the given names from a codebase URL path.

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- - ### Method Detail

    - #### loadClass

          @Deprecated
          public static Class<?> loadClass(String name)
                                                throws MalformedURLException,
                                                       ClassNotFoundException

      Deprecated. replaced by `loadClass(String,String)` method
      Loads the class with the specified `name`.

      This method delegates to `loadClass(String,String)`, passing `null` as the first argument and `name` as the second argument.

      Parameters:
        `name` - the name of the class to load
      Returns:
        the `Class` object representing the loaded class
      Throws:
        `MalformedURLException` - if a provider-specific URL used to load classes is invalid
        `ClassNotFoundException` - if a definition for the class could not be found at the codebase location
      See Also:
        `loadClass(String,String)`

    - #### loadClass

          public static Class<?> loadClass(URL codebase,
                                           String name)
                                    throws MalformedURLException,
                                           ClassNotFoundException

      Loads a class from a codebase URL. If `codebase` is `null`, then this method will behave the same as `loadClass(String,String)` with a `null` `codebase` and the given class name.

      This method delegates to the `RMIClassLoaderSpi.loadClass(String,String,ClassLoader)` method of the provider instance, passing the result of invoking `URL.toString()` on the given URL (or `null` if `codebase` is null) as the first argument, `name` as the second argument, and `null` as the third argument.

      Parameters:
        `codebase` - the URL to load the class from, or `null`
        `name` - the name of the class to load
      Returns:
        the `Class` object representing the loaded class
      Throws:
        `MalformedURLException` - if `codebase` is `null` and a provider-specific URL used to load classes is invalid
        `ClassNotFoundException` - if a definition for the class could not be found at the specified URL

    - #### loadClass

          public static Class<?> loadClass(String codebase,
                                           String name)
                                    throws MalformedURLException,
                                           ClassNotFoundException

      Loads a class from a codebase URL path.

      This method delegates to the `RMIClassLoaderSpi.loadClass(String,String,ClassLoader)` method of the provider instance, passing `codebase` as the first argument, `name` as the second argument, and `null` as the third argument.

      Parameters:
        `codebase` - the list of URLs (separated by spaces) to load the class from, or `null`
        `name` - the name of the class to load
      Returns:
        the `Class` object representing the loaded class
      Throws:
        `MalformedURLException` - if `codebase` is non-`null` and contains an invalid URL, or if `codebase` is `null` and a provider-specific URL used to load classes is invalid
        `ClassNotFoundException` - if a definition for the class could not be found at the specified location
      Since:
        1.2

    - #### loadClass

          public static Class<?> loadClass(String codebase,
                                           String name,
                                           ClassLoader defaultLoader)
                                    throws MalformedURLException,
                                           ClassNotFoundException

      Loads a class from a codebase URL path, optionally using the supplied loader. This method should be used when the caller would like to make available to the provider implementation an additional contextual class loader to consider, such as the loader of a caller on the stack. Typically, a provider implementation will attempt to resolve the named class using the given `defaultLoader`, if specified, before attempting to resolve the class from the codebase URL path.

      This method delegates to the `RMIClassLoaderSpi.loadClass(String,String,ClassLoader)` method of the provider instance, passing `codebase` as the first argument, `name` as the second argument, and `defaultLoader` as the third argument.

      Parameters:
        `codebase` - the list of URLs (separated by spaces) to load the class from, or `null`
        `name` - the name of the class to load
        `defaultLoader` - additional contextual class loader to use, or `null`
      Returns:
        the `Class` object representing the loaded class
      Throws:
        `MalformedURLException` - if `codebase` is non-`null` and contains an invalid URL, or if `codebase` is `null` and a provider-specific URL used to load classes is invalid
        `ClassNotFoundException` - if a definition for the class could not be found at the specified location
      Since:
        1.4

    - #### loadProxyClass

          public static Class<?> loadProxyClass(String codebase,
                                                String[] interfaces,
                                                ClassLoader defaultLoader)
                                         throws ClassNotFoundException,
                                                MalformedURLException

      Loads a dynamic proxy class (see `Proxy`) that implements a set of interfaces with the given names from a codebase URL path.

      The interfaces will be resolved similar to classes loaded via the `loadClass(String,String)` method using the given `codebase`.

      This method delegates to the `RMIClassLoaderSpi.loadProxyClass(String,String[],ClassLoader)` method of the provider instance, passing `codebase` as the first argument, `interfaces` as the second argument, and `defaultLoader` as the third argument.

      Parameters:
        `codebase` - the list of URLs (space-separated) to load classes from, or `null`
        `interfaces` - the names of the interfaces for the proxy class to implement
        `defaultLoader` - additional contextual class loader to use, or `null`
      Returns:
        a dynamic proxy class that implements the named interfaces
      Throws:
        `MalformedURLException` - if `codebase` is non-`null` and contains an invalid URL, or if `codebase` is `null` and a provider-specific URL used to load classes is invalid
        `ClassNotFoundException` - if a definition for one of the named interfaces could not be found at the specified location, or if creation of the dynamic proxy class failed (such as if `Proxy.getProxyClass(ClassLoader,Class[])` would throw an `IllegalArgumentException` for the given interface list)
      Since:
        1.4

    - #### getClassLoader

          public static ClassLoader getClassLoader(String codebase)
                                            throws MalformedURLException,
                                                   SecurityException

      Returns a class loader that loads classes from the given codebase URL path.

      The class loader returned is the class loader that the `loadClass(String,String)` method would use to load classes for the same `codebase` argument.

      This method delegates to the `RMIClassLoaderSpi.getClassLoader(String)` method of the provider instance, passing `codebase` as the argument.

      If there is a security manger, its `checkPermission` method will be invoked with a `RuntimePermission("getClassLoader")` permission; this could result in a `SecurityException`. The provider implementation of this method may also perform further security checks to verify that the calling context has permission to connect to all of the URLs in the codebase URL path.

      Parameters:
        `codebase` - the list of URLs (space-separated) from which the returned class loader will load classes from, or `null`
      Returns:
        a class loader that loads classes from the given codebase URL path
      Throws:
        `MalformedURLException` - if `codebase` is non-`null` and contains an invalid URL, or if `codebase` is `null` and a provider-specific URL used to identify the class loader is invalid
        `SecurityException` - if there is a security manager and the invocation of its `checkPermission` method fails, or if the caller does not have permission to connect to all of the URLs in the codebase URL path
      Since:
        1.3

    - #### getClassAnnotation

          public static String getClassAnnotation(Class<?> cl)

      Returns the annotation string (representing a location for the class definition) that RMI will use to annotate the class descriptor when marshalling objects of the given class.

      This method delegates to the `RMIClassLoaderSpi.getClassAnnotation(Class)` method of the provider instance, passing `cl` as the argument.

      Parameters:
        `cl` - the class to obtain the annotation for
      Returns:
        a string to be used to annotate the given class when it gets marshalled, or `null`
      Throws:
        `NullPointerException` - if `cl` is `null`
      Since:
        1.2

    - #### getDefaultProviderInstance

          public static RMIClassLoaderSpi getDefaultProviderInstance()

      Returns the canonical instance of the default provider for the service provider interface `RMIClassLoaderSpi`. If the system property `java.rmi.server.RMIClassLoaderSpi` is not defined, then the `RMIClassLoader` static methods
      - `loadClass(URL,String)`
      - `loadClass(String,String)`
      - `loadClass(String,String,ClassLoader)`
      - `loadProxyClass(String,String[],ClassLoader)`
      - `getClassLoader(String)`
      - `getClassAnnotation(Class)`
      will use the canonical instance of the default provider as the service provider instance.

      If there is a security manager, its `checkPermission` method will be invoked with a `RuntimePermission("setFactory")` permission; this could result in a `SecurityException`.

      The default service provider instance implements `RMIClassLoaderSpi` as follows:

      > The `getClassAnnotation` method returns a `String` representing the codebase URL path that a remote party should use to download the definition for the specified class. The format of the returned string is a path of URLs separated by spaces. The codebase string returned depends on the defining class loader of the specified class:

      > - If the class loader is the system class loader (see `ClassLoader.getSystemClassLoader()`), a parent of the system class loader such as the loader used for installed extensions, or the bootstrap class loader (which may be represented by `null`), then the value of the `java.rmi.server.codebase` property (or possibly an earlier cached value) is returned, or `null` is returned if that property is not set.

      > - Otherwise, if the class loader is an instance of `URLClassLoader`, then the returned string is a space-separated list of the external forms of the URLs returned by invoking the `getURLs` methods of the loader. If the `URLClassLoader` was created by this provider to service an invocation of its `loadClass` or `loadProxyClass` methods, then no permissions are required to get the associated codebase string. If it is an arbitrary other `URLClassLoader` instance, then if there is a security manager, its `checkPermission` method will be invoked once for each URL returned by the `getURLs` method, with the permission returned by invoking `openConnection().getPermission()` on each URL; if any of those invocations throws a `SecurityException` or an `IOException`, then the value of the `java.rmi.server.codebase` property (or possibly an earlier cached value) is returned, or `null` is returned if that property is not set.

      > - Finally, if the class loader is not an instance of `URLClassLoader`, then the value of the `java.rmi.server.codebase` property (or possibly an earlier cached value) is returned, or `null` is returned if that property is not set.

      > For the implementations of the methods described below, which all take a `String` parameter named `codebase` that is a space-separated list of URLs, each invocation has an associated codebase loader that is identified using the `codebase` argument in conjunction with the current thread's context class loader (see `Thread.getContextClassLoader()`). When there is a security manager, this provider maintains an internal table of class loader instances (which are at least instances of `URLClassLoader`) keyed by the pair of their parent class loader and their codebase URL path (an ordered list of URLs). If the `codebase` argument is `null`, the codebase URL path is the value of the system property `java.rmi.server.codebase` or possibly an earlier cached value. For a given codebase URL path passed as the `codebase` argument to an invocation of one of the below methods in a given context, the codebase loader is the loader in the table with the specified codebase URL path and the current thread's context class loader as its parent. If no such loader exists, then one is created and added to the table. The table does not maintain strong references to its contained loaders, in order to allow them and their defined classes to be garbage collected when not otherwise reachable. In order to prevent arbitrary untrusted code from being implicitly loaded into a virtual machine with no security manager, if there is no security manager set, the codebase loader is just the current thread's context class loader (the supplied codebase URL path is ignored, so remote class loading is disabled).

      > The `getClassLoader` method returns the codebase loader for the specified codebase URL path. If there is a security manager, then if the calling context does not have permission to connect to all of the URLs in the codebase URL path, a `SecurityException` will be thrown.

      > The `loadClass` method attempts to load the class with the specified name as follows:

      > > If the `defaultLoader` argument is non-`null`, it first attempts to load the class with the specified `name` using the `defaultLoader`, such as by evaluating

      > >          Class.forName(name, false, defaultLoader)

      > > If the class is successfully loaded from the `defaultLoader`, that class is returned. If an exception other than `ClassNotFoundException` is thrown, that exception is thrown to the caller.

      > > Next, the `loadClass` method attempts to load the class with the specified `name` using the codebase loader for the specified codebase URL path. If there is a security manager, then the calling context must have permission to connect to all of the URLs in the codebase URL path; otherwise, the current thread's context class loader will be used instead of the codebase loader.

      > The `loadProxyClass` method attempts to return a dynamic proxy class with the named interface as follows:

      > > If the `defaultLoader` argument is non-`null` and all of the named interfaces can be resolved through that loader, then,

      > > - if all of the resolved interfaces are `public`, then it first attempts to obtain a dynamic proxy class (using `Proxy.getProxyClass`) for the resolved interfaces defined in the codebase loader; if that attempt throws an `IllegalArgumentException`, it then attempts to obtain a dynamic proxy class for the resolved interfaces defined in the `defaultLoader`. If both attempts throw `IllegalArgumentException`, then this method throws a `ClassNotFoundException`. If any other exception is thrown, that exception is thrown to the caller.
      > > - if all of the non-`public` resolved interfaces are defined in the same class loader, then it attempts to obtain a dynamic proxy class for the resolved interfaces defined in that loader.
      > > - otherwise, a `LinkageError` is thrown (because a class that implements all of the specified interfaces cannot be defined in any loader).

      > > Otherwise, if all of the named interfaces can be resolved through the codebase loader, then,

      > > - if all of the resolved interfaces are `public`, then it attempts to obtain a dynamic proxy class for the resolved interfaces in the codebase loader. If the attempt throws an `IllegalArgumentException`, then this method throws a `ClassNotFoundException`.
      > > - if all of the non-`public` resolved interfaces are defined in the same class loader, then it attempts to obtain a dynamic proxy class for the resolved interfaces defined in that loader.
      > > - otherwise, a `LinkageError` is thrown (because a class that implements all of the specified interfaces cannot be defined in any loader).

      > > Otherwise, a `ClassNotFoundException` is thrown for one of the named interfaces that could not be resolved.

      Returns:
        the canonical instance of the default service provider
      Throws:
        `SecurityException` - if there is a security manager and the invocation of its `checkPermission` method fails
      Since:
        1.4

    - #### getSecurityContext

          @Deprecated
          public static Object getSecurityContext(ClassLoader loader)

      Deprecated. no replacement. As of the Java 2 platform v1.2, RMI no longer uses this method to obtain a class loader's security context.
      Returns the security context of the given class loader.
      Parameters:
        `loader` - a class loader from which to get the security context
      Returns:
        the security context
      See Also:
        `SecurityManager.getSecurityContext()`

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