objectfactory.java

java1.6众多例子参考

                    if (fis != null) {
                        try {
                            fis.close();
                        }
                        // Ignore the exception.
                        catch (IOException exc) {}
                    }
                }	            
            }
            if(fXalanProperties != null) {
                factoryClassName = fXalanProperties.getProperty(factoryId);
            }
        } else {
            FileInputStream fis = null;
            try {
                fis = ss.getFileInputStream(new File(propertiesFilename));
                Properties props = new Properties();
                props.load(fis);
                factoryClassName = props.getProperty(factoryId);
            } catch (Exception x) {
                // assert(x instanceof FileNotFoundException
                //        || x instanceof SecurityException)
                // In both cases, ignore and continue w/ next location
            }
            finally {
                // try to close the input stream if one was opened.
                if (fis != null) {
                    try {
                        fis.close();
                    }
                    // Ignore the exception.
                    catch (IOException exc) {}
                }
            }               
        }
        if (factoryClassName != null) {
            debugPrintln("found in " + propertiesFilename + ", value="
                          + factoryClassName);
            return factoryClassName;
        }

        // Try Jar Service Provider Mechanism
        return findJarServiceProviderName(factoryId);
    } // lookUpFactoryClass(String,String):String

    //
    // Private static methods
    //

    /** Prints a message to standard error if debugging is enabled. */
    private static void debugPrintln(String msg) {
        if (DEBUG) {
            System.err.println("JAXP: " + msg);
        }
    } // debugPrintln(String)

    /**
     * Figure out which ClassLoader to use.  For JDK 1.2 and later use
     * the context ClassLoader.
     */
    static ClassLoader findClassLoader()
        throws ConfigurationError
    { 
        SecuritySupport ss = SecuritySupport.getInstance();

        // Figure out which ClassLoader to use for loading the provider
        // class.  If there is a Context ClassLoader then use it.
        ClassLoader context = ss.getContextClassLoader();
        ClassLoader system = ss.getSystemClassLoader();

        ClassLoader chain = system;
        while (true) {
            if (context == chain) {
                // Assert: we are on JDK 1.1 or we have no Context ClassLoader
                // or any Context ClassLoader in chain of system classloader
                // (including extension ClassLoader) so extend to widest
                // ClassLoader (always look in system ClassLoader if Xalan
                // is in boot/extension/system classpath and in current
                // ClassLoader otherwise); normal classloaders delegate
                // back to system ClassLoader first so this widening doesn't
                // change the fact that context ClassLoader will be consulted
                ClassLoader current = ObjectFactory.class.getClassLoader();

                chain = system;
                while (true) {
                    if (current == chain) {
                        // Assert: Current ClassLoader in chain of
                        // boot/extension/system ClassLoaders
                        return system;
                    }
                    if (chain == null) {
                        break;
                    }
                    chain = ss.getParentClassLoader(chain);
                }

                // Assert: Current ClassLoader not in chain of
                // boot/extension/system ClassLoaders
                return current;
            }

            if (chain == null) {
                // boot ClassLoader reached
                break;
            }

            // Check for any extension ClassLoaders in chain up to
            // boot ClassLoader
            chain = ss.getParentClassLoader(chain);
        };

        // Assert: Context ClassLoader not in chain of
        // boot/extension/system ClassLoaders
        return context;
    } // findClassLoader():ClassLoader

    /**
     * Create an instance of a class using the specified ClassLoader
     */ 
    static Object newInstance(String className, ClassLoader cl,
                                      boolean doFallback)
        throws ConfigurationError
    {
        // assert(className != null);
        try{
            Class providerClass = findProviderClass(className, cl, doFallback);
            Object instance = providerClass.newInstance();
            debugPrintln("created new instance of " + providerClass +
                   " using ClassLoader: " + cl);
            return instance;
        } catch (ClassNotFoundException x) {
            throw new ConfigurationError(
                "Provider " + className + " not found", x);
        } catch (Exception x) {
            throw new ConfigurationError(
                "Provider " + className + " could not be instantiated: " + x,
                x);
        }
    }

    /**
     * Find a Class using the specified ClassLoader
     */ 
    static Class findProviderClass(String className, ClassLoader cl,
                                           boolean doFallback)
        throws ClassNotFoundException, ConfigurationError
    {   
        //throw security exception if the calling thread is not allowed to access the
        //class. Restrict the access to the package classes as specified in java.security policy.
        SecurityManager security = System.getSecurityManager();
        try{
                if (security != null){
                    final int lastDot = className.lastIndexOf(".");
                    String packageName = className;
                    if (lastDot != -1) packageName = className.substring(0, lastDot);
                    security.checkPackageAccess(packageName);
                 }   
        }catch(SecurityException e){
            throw e;
        }
        
        Class providerClass;
        if (cl == null) {
            // XXX Use the bootstrap ClassLoader.  There is no way to
            // load a class using the bootstrap ClassLoader that works
            // in both JDK 1.1 and Java 2.  However, this should still
            // work b/c the following should be true:
            //
            // (cl == null) iff current ClassLoader == null
            //
            // Thus Class.forName(String) will use the current
            // ClassLoader which will be the bootstrap ClassLoader.
            providerClass = Class.forName(className);
        } else {
            try {
                providerClass = cl.loadClass(className);
            } catch (ClassNotFoundException x) {
                if (doFallback) {
                    // Fall back to current classloader
                    ClassLoader current = ObjectFactory.class.getClassLoader();
                    if (current == null) {
                        providerClass = Class.forName(className);
                    } else if (cl != current) {
                        cl = current;
                        providerClass = cl.loadClass(className);
                    } else {
                        throw x;
                    }
                } else {
                    throw x;
                }
            }
        }

        return providerClass;
    }

    /**
     * Find the name of service provider using Jar Service Provider Mechanism
     *
     * @return instance of provider class if found or null
     */
    private static String findJarServiceProviderName(String factoryId)
    {
        SecuritySupport ss = SecuritySupport.getInstance();
        String serviceId = SERVICES_PATH + factoryId;
        InputStream is = null;

        // First try the Context ClassLoader
        ClassLoader cl = findClassLoader();

        is = ss.getResourceAsStream(cl, serviceId);

        // If no provider found then try the current ClassLoader
        if (is == null) {
            ClassLoader current = ObjectFactory.class.getClassLoader();
            if (cl != current) {
                cl = current;
                is = ss.getResourceAsStream(cl, serviceId);
            }
        }

        if (is == null) {
            // No provider found
            return null;
        }

        debugPrintln("found jar resource=" + serviceId +
               " using ClassLoader: " + cl);

        // Read the service provider name in UTF-8 as specified in
        // the jar spec.  Unfortunately this fails in Microsoft
        // VJ++, which does not implement the UTF-8
        // encoding. Theoretically, we should simply let it fail in
        // that case, since the JVM is obviously broken if it
        // doesn't support such a basic standard.  But since there
        // are still some users attempting to use VJ++ for
        // development, we have dropped in a fallback which makes a
        // second attempt using the platform's default encoding. In
        // VJ++ this is apparently ASCII, which is a subset of
        // UTF-8... and since the strings we'll be reading here are
        // also primarily limited to the 7-bit ASCII range (at
        // least, in English versions), this should work well
        // enough to keep us on the air until we're ready to
        // officially decommit from VJ++. [Edited comment from
        // jkesselm]
        BufferedReader rd;
        try {
            rd = new BufferedReader(new InputStreamReader(is, "UTF-8"));
        } catch (java.io.UnsupportedEncodingException e) {
            rd = new BufferedReader(new InputStreamReader(is));
        }
        
        String factoryClassName = null;
        try {
            // XXX Does not handle all possible input as specified by the
            // Jar Service Provider specification
            factoryClassName = rd.readLine();
        } catch (IOException x) {
            // No provider found
            return null;
        }
        finally {
            try {
                // try to close the reader.
                rd.close();
            }
            // Ignore the exception.
            catch (IOException exc) {}
        }          

        if (factoryClassName != null &&
            ! "".equals(factoryClassName)) {
            debugPrintln("found in resource, value="
                   + factoryClassName);

            // Note: here we do not want to fall back to the current
            // ClassLoader because we want to avoid the case where the
            // resource file was found using one ClassLoader and the
            // provider class was instantiated using a different one.
            return factoryClassName;
        }

        // No provider found
        return null;
    }

    //
    // Classes
    //

    /**
     * A configuration error.
     */
    static class ConfigurationError 
        extends Error {
                static final long serialVersionUID = -2293620736651286953L;
        //
        // Data
        //

        /** Exception. */
        private Exception exception;

        //
        // Constructors
        //

        /**
         * Construct a new instance with the specified detail string and
         * exception.
         */
        ConfigurationError(String msg, Exception x) {
            super(msg);
            this.exception = x;
        } // <init>(String,Exception)

        //
        // Public methods
        //

        /** Returns the exception associated to this error. */
        Exception getException() {
            return exception;
        } // getException():Exception

    } // class ConfigurationError

} // class ObjectFactory