src/main/java/org/apache/sling/scripting/java/impl/ServletWrapper.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.sling.scripting.java.impl;

import java.util.List;

import javax.servlet.Servlet;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.SingleThreadModel;
import javax.servlet.UnavailableException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.apache.sling.commons.classloader.DynamicClassLoader;
import org.apache.sling.commons.compiler.CompilationResult;
import org.apache.sling.commons.compiler.CompilerMessage;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * This class wraps the java servlet and handles its compilation,
 * instantiation, invocation und destruction.
 */
public class ServletWrapper {

    /** The logger. */
    private final Logger logger = LoggerFactory.getLogger(getClass());

    /** The servlet config. */
    private final ServletConfig config;

    /** Sling IO Provider. */
    private final SlingIOProvider ioProvider;

    /** The name of the generated class. */
    private final String className;

    /** The path to the servlet. */
    private final String sourcePath;

    /** Flag for handling modifications. */
    private volatile long lastModificationTest = 0L;

    /** The compiled and instantiated servlet. */
    private volatile Servlet theServlet;

    /** Flag handling an unavailable exception. */
    private volatile long available = 0L;

    /** The exception thrown by the compilation. */
    private volatile Exception compileException;

    /**
     * A wrapper for servlets.
     */
    public ServletWrapper(final ServletConfig config,
                          final SlingIOProvider ioProvider,
                          final String servletPath) {
        this.config = config;
        this.ioProvider = ioProvider;
        this.sourcePath = servletPath;
        this.className = CompilerUtil.mapSourcePath(this.sourcePath).substring(1).replace('/', '.');
    }

    /**
     * Call the servlet.
     * @param request The current request.
     * @param response The current response.
     * @throws Exception
     */
    public void service(HttpServletRequest request,
                         HttpServletResponse response)
    throws Exception {
        try {
            if ((available > 0L) && (available < Long.MAX_VALUE)) {
                if (available > System.currentTimeMillis()) {
                    response.setDateHeader("Retry-After", available);
                    response.sendError
                        (HttpServletResponse.SC_SERVICE_UNAVAILABLE,
                         "Servlet unavailable.");
                    logger.error("Java servlet {} is unavailable.", this.sourcePath);
                    return;
                }
                // Wait period has expired. Reset.
                available = 0;
            }

            // check for compilation
            if (this.lastModificationTest <= 0 ) {
                synchronized (this) {
                    if (this.lastModificationTest <= 0 ) {
                        this.compile();
                    } else if (compileException != null) {
                        // Throw cached compilation exception
                        throw compileException;
                    }
                }
            } else if (compileException != null) {
                // Throw cached compilation exception
                throw compileException;
            }

            final Servlet servlet = getServlet();

            // invoke the servlet
            if (servlet instanceof SingleThreadModel) {
                // sync on the wrapper so that the freshness
                // of the page is determined right before servicing
                synchronized (this) {
                    servlet.service(request, response);
                }
            } else {
                servlet.service(request, response);
            }

        } catch (UnavailableException ex) {
            int unavailableSeconds = ex.getUnavailableSeconds();
            if (unavailableSeconds <= 0) {
                unavailableSeconds = 60;        // Arbitrary default
            }
            available = System.currentTimeMillis() +
                (unavailableSeconds * 1000L);
            response.sendError
                (HttpServletResponse.SC_SERVICE_UNAVAILABLE,
                 ex.getMessage());
            logger.error("Java servlet {} is unavailable.", this.sourcePath);
        }
    }

    /**
     * Destroy the servlet.
     */
    public void destroy() {
        if (theServlet != null) {
            theServlet.destroy();
            theServlet = null;
        }
    }

    /**
     * Handle the modification.
     */
    public void handleModification() {
        logger.debug("Received modification event for {}", this.sourcePath);
        this.lastModificationTest = -1;
    }

    /**
     * Check if the used classloader is still valid
     */
    private boolean checkReload() {
        final Servlet servlet = this.theServlet;
        final Class<?> servletClass  = servlet != null ? servlet.getClass() : null;
        if ( servletClass != null && servletClass.getClassLoader() instanceof DynamicClassLoader ) {
            return !((DynamicClassLoader)servletClass.getClassLoader()).isLive();
        }
        return false;
    }

    /**
     * Get the servlet class - if the used classloader is not valid anymore
     * the class is reloaded.
     */
    public Servlet getServlet()
    throws Exception {
        // check if the used class loader is still alive
        if (this.checkReload()) {
            synchronized (this) {
                if (this.checkReload()) {
                    this.compile();
                }
            }
        }
        return theServlet;
    }

    /**
     * Compile the servlet java class
     * and instantiate the servlet
     */
    private void compile()
    throws Exception {
        logger.debug("Compiling {}", this.sourcePath);
        // clear exception
        this.compileException = null;
        try {
            final CompilerOptions opts = (this.lastModificationTest == -1 ? this.ioProvider.getForceCompileOptions() : this.ioProvider.getOptions());
            final CompilationUnit unit = new CompilationUnit(this.sourcePath, className, ioProvider);
            final CompilationResult result = this.ioProvider.getCompiler().compile(new org.apache.sling.commons.compiler.CompilationUnit[] {unit},
                    opts);

            final List<CompilerMessage> errors = result.getErrors();
            if ( errors != null && errors.size() > 0 ) {
                throw CompilerException.create(errors, this.sourcePath);
            }
            if ( result.didCompile() || this.theServlet == null ) {
                destroy();
                final Class<?> servletClass = result.loadCompiledClass(this.className);
                final Servlet servlet = (Servlet) servletClass.newInstance();
                servlet.init(this.config);

                this.theServlet = servlet;
            }
        } catch (final Exception ex) {
            // store exception for futher access attempts
            this.compileException = ex;
            throw ex;
        } finally {
            this.lastModificationTest = System.currentTimeMillis();
        }
    }

    /** Compiler exception .*/
    protected final static class CompilerException extends ServletException {

        private static final long serialVersionUID = 7353686069328527452L;

        public static CompilerException create(final List<CompilerMessage> errors,
                final String fileName) {
            final StringBuilder buffer = new StringBuilder();
            buffer.append("Compilation errors in ");
            buffer.append(fileName);
            buffer.append(":\n");
            for(final CompilerMessage e : errors) {
                buffer.append("Line ");
                buffer.append(e.getLine());
                buffer.append(", column ");
                buffer.append(e.getColumn());
                buffer.append(" : " );
                buffer.append(e.getMessage());
                buffer.append("\n");
            }
            return new CompilerException(buffer.toString());
        }

        public CompilerException(final String message) {
           super(message);
        }
    }
}