Module  java.rmi
软件包  java.rmi.server

Class RMIClassLoader


  • public class RMIClassLoader
extends Object
    RMIClassLoader包含静态方法来支持使用RMI的动态类加载。 包括从网络位置（一个或多个URL）加载类的方法，并获取远程方应由现有类加载的位置。 这些方法由RMI运行时使用，当编组和解组合包含在远程方法调用的参数和返回值中时，它们也可以被应用程序直接调用，以模仿RMI的动态类加载行为。

    执行以下静态方法

    由RMIClassLoaderSpi的实例提供，这些方法的服务提供程序接口。 当调用其中一种方法时，其行为是委托给服务提供者实例上的相应方法。 每个方法委托给提供者实例的详细信息，请参见每个特定方法的文档。

    服务提供商实例选择如下：

    • 如果定义了系统属性java.rmi.server.RMIClassLoaderSpi ，那么如果其值等于字符串"default" ，则提供者实例将是通过调用getDefaultProviderInstance()方法返回的值，对于任何其他值，如果使用属性值命名的类可以由系统类加载器加载（参见ClassLoader.getSystemClassLoader() ），该类可分配给RMIClassLoaderSpi并具有公共无参数构造函数，那么将调用该构造函数来创建提供者实例。 如果属性被定义，但这些条件中的任何其他条件都不为真，那么未指定的Error将被抛出到尝试使用RMIClassLoader代码中，表示无法获取提供者实例。
    • 如果名为META-INF/services/java.rmi.server.RMIClassLoaderSpi的资源对于系统类加载器是可见的，则该资源的内容将被解释为提供者配置文件，并且该文件中指定的第一个类名被用作提供者类名称。 如果具有该名称的类可以由系统类加载器加载，并且该类可以分配给RMIClassLoaderSpi并具有公共无参构造函数，则将调用该构造函数来创建提供者实例。 如果找到资源，但提供程序不能如所述实例化，那么未指定的Error将被抛出到尝试使用RMIClassLoader代码，指示无法获取提供程序实例。
    • 否则，提供者实例将是通过调用getDefaultProviderInstance()方法返回的值。
    从以下版本开始：
    1.1
    另请参见：
    RMIClassLoaderSpi

    • 方法详细信息

      • loadClass

        public static Class<?> loadClass​(String codebase,
                                 String name,
                                 ClassLoader defaultLoader)
                          throws MalformedURLException,
                                 ClassNotFoundException
        从代码库URL路径加载类，可选地使用提供的加载程序。 当调用者想要向提供者实现提供一个要考虑的附加上下文类加载器时，应该使用此方法，例如堆栈上的调用者的加载程序。 通常，在尝试从代码库URL路径解析类之前，提供者实现将尝试使用给定的defaultLoader （如果指定）来解析命名类。

        此方法委托给RMIClassLoaderSpi.loadClass(String,String,ClassLoader)提供程序实例的方法，传递codebase作为第一个参数， name作为第二个参数，和defaultLoader作为第三个参数。

        参数
        codebase - 加载类的URL列表（由空格分隔），或 null
        name - 要加载的类的名称
        defaultLoader - 要使用的附加上下文类加载器，或 null
        结果
        表示加载类的 Class对象
        异常
        MalformedURLException - 如果 codebase不是 null并且包含无效的URL，或者如果 codebase是 null并且用于加载类的提供商特定的URL无效
        ClassNotFoundException - 如果在指定位置找不到类的定义
        从以下版本开始：
        1.4

      • loadProxyClass

        public static Class<?> loadProxyClass​(String codebase,
                                      String[] interfaces,
                                      ClassLoader defaultLoader)
                               throws ClassNotFoundException,
                                      MalformedURLException
        加载动态代理类（请参阅Proxy ），该代码类从代码库URL路径中实现了一组具有给定名称的接口。

        接口将被解析为类似于使用给定的codebase通过loadClass(String,String)方法加载的类。

        此方法委托给提供者实例的RMIClassLoaderSpi.loadProxyClass(String,String[],ClassLoader)方法，作为第二个参数传递codebase作为第二个参数， interfaces作为第二个参数， defaultLoader作为第三个参数。

        参数
        codebase - 加载类的URL列表（空格分隔），或 null
        interfaces - 代理类实现的接口的名称
        defaultLoader - 要使用的附加上下文类加载器，或 null
        结果
        一个实现命名接口的动态代理类
        异常
        MalformedURLException - 如果 codebase不是 null并且包含无效的URL，或者如果 codebase是 null并且用于加载类的提供者特定URL无效
        ClassNotFoundException - 如果在指定位置找不到某个命名接口的定义，或者如果动态代理类的创建失败（例如，如果 Proxy.getProxyClass(ClassLoader,Class[])将为给定接口列表抛出一个 IllegalArgumentException ）
        从以下版本开始：
        1.4

      • getClassLoader

        public static ClassLoader getClassLoader​(String codebase)
                                  throws MalformedURLException,
                                         SecurityException
        返回从给定的代码库URL路径加载类的类加载器。

        返回的类加载器是loadClass(String,String)方法用于为相同的codebase参数加载类的类加载器。

        该方法委托给提供者实例的RMIClassLoaderSpi.getClassLoader(String)方法，传递codebase作为参数。

        如果有一个安全管理员，它的checkPermission方法将被调用一个RuntimePermission("getClassLoader")权限; 这可能会导致一个SecurityException 。 该方法的提供者实现还可以执行进一步的安全检查，以验证呼叫上下文是否具有连接到代码库URL路径中的所有URL的权限。

        参数
        codebase - 返回的类加载器从其加载类的URL（空格分隔）列表，或 null
        结果
        一个类加载器，它从给定的代码库URL路径加载类
        异常
        MalformedURLException - 如果 codebase不是 null并且包含无效的URL，或者如果 codebase是 null并且用于标识类加载器的提供者特定URL无效
        SecurityException - 如果有安全管理器，并且其 checkPermission方法的调用失败，或者主叫方没有权限连接到代码库URL路径中的所有URL
        从以下版本开始：
        1.3

      • getClassAnnotation

        public static String getClassAnnotation​(Class<?> cl)
        返回在编组给定类的对象时，RMI将用于注释类描述符的注释字符串（表示类定义的位置）。

        该方法委托给提供者实例的RMIClassLoaderSpi.getClassAnnotation(Class)方法，作为参数传递cl 。

        参数
        cl - 获取注释的类
        结果
        一个字符串，用于在给定的类进行编组时用于注释，或者是 null
        异常
        NullPointerException - 如果 cl是 null
        从以下版本开始：
        1.2

      • getDefaultProviderInstance

        public static RMIClassLoaderSpi getDefaultProviderInstance​()
        返回服务提供商接口RMIClassLoaderSpi的默认提供程序的规范实例。 如果系统属性java.rmi.server.RMIClassLoaderSpi没有定义，那么RMIClassLoader静态方法 将使用默认提供者的规范实例作为服务提供者实例。

        如果有安全管理员，其checkPermission方法将被调用RuntimePermission("setFactory")权限; 这可能会导致一个SecurityException 。

        默认服务提供商实例实现RMIClassLoaderSpi如下：

        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.

        结果
        默认服务提供商的规范实例
        异常
        SecurityException - 如果有安全管理器，并且其 checkPermission方法的调用失败
        从以下版本开始：
        1.4

      • getSecurityContext

        @Deprecated
public static Object getSecurityContext​(ClassLoader loader)
        已过时。 没有替换。 从Java 2平台v1.2开始，RMI不再使用此方法来获取类加载器的安全上下文。
        返回给定类加载器的安全上下文。
        参数
        loader - 从中获取安全上下文的类加载器
        结果
        安全上下文
        另请参见：
        SecurityManager.getSecurityContext()