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DWR is easy Ajax for Java. It makes it simple to call Java code directly from Javascript. It gets rid of almost all the boiler plate code between the web browser and your Java code.
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/*
 * Copyright 2007 Tim Peierls
 *
 * Licensed 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.directwebremoting.guice;

import java.util.List;
import java.util.concurrent.atomic.AtomicLong;

import org.directwebremoting.AjaxFilter;
import org.directwebremoting.extend.Converter;
import org.directwebremoting.guice.util.AbstractModule;
import org.directwebremoting.util.LocalUtil;

import com.google.inject.binder.ConstantBindingBuilder;
import com.google.inject.binder.LinkedBindingBuilder;

import static java.util.Arrays.*;

import static org.directwebremoting.guice.ParamName.*;

/**
 * An extension of the enhanced {@link AbstractModule} from the util
 * subpackage that adds DWR configuration methods when used in conjunction
 * with {@link DwrGuiceServlet}.
 * @author Tim Peierls [tim at peierls dot net]
 */
public abstract class AbstractDwrModule extends AbstractModule
{
    /**
     * Implement this method to configure Guice bindings for a DWR-based
     * web application.
     */
    @Override
    protected abstract void configure();


    /**
     * Call this method before configuration to explicitly determine the behavior
     * of {@link #bindDwrScopes()}. If not called, the default behavior is to bind
     * the potentially conflicting types only if the Guice ServletModule is not found
     * in the classloader. That is usually the right behavior; it should not often be
     * necessary to call this method.
     * @param newBindPotentiallyConflictingTypes whether to bind request, response, and
     *   session types to DWR scopes without qualifying with an annotation
     */
    protected final void bindPotentiallyConflictingTypes(boolean newBindPotentiallyConflictingTypes)
    {
        this.bindPotentiallyConflictingTypes = newBindPotentiallyConflictingTypes;
    }

    /**
     * Configure DWR scopes and bindings for servlet-related types.
     * If {@link #bindPotentiallyConflictingTypes} has been
     * called previously for this module, this method is equivalent
     * to calling {@link #bindDwrScopes(boolean) bindDwrScopes}
     * with the value passed to {@link #bindPotentiallyConflictingTypes}.
     * Otherwise this module will include bindings that might conflict
     * with those provided by Guice's ServletModule only
     * if ServletModule is not found in the current class loader.
     * Idempotent within current thread.
     */
    protected void bindDwrScopes()
    {
        if (this.bindPotentiallyConflictingTypes == null)
        {
            bindDwrScopes(!guiceServletModuleExists());
        }
        else
        {
            bindDwrScopes(this.bindPotentiallyConflictingTypes);
        }
    }

    /**
     * Configure DWR scopes and bindings for servlet-related types,
     * specifying explicitly whether to include bindings that might
     * conflict with those provided by Guice's ServletModule.
     * The {@link #bindDwrScopes() variant} of this method that takes
     * no arguments usually does the right; it should not often be
     * necessary to call this method.
     * Idempotent within current thread.
     * @param newBindPotentiallyConflictingTypes whether to bind request, response,
     *     and session types (risking conflict with Guice)
     */
    protected void bindDwrScopes(boolean newBindPotentiallyConflictingTypes)
    {
        if (!boundDwrScopes.get())
        {
            boundDwrScopes.set(true);
            install(new DwrGuiceServletModule(newBindPotentiallyConflictingTypes));
        }
    }


    /**
     * Call this method in
     * {@link org.directwebremoting.guice.AbstractDwrModule#configure configure}
     * to specify classes that DWR should scan for annotations.
     * @param classes the classes to be scanned for DWR-specific annotations
     */
    protected void bindAnnotatedClasses(Class... classes)
    {
        bind(List.class)
            .annotatedWith(new InitParamImpl(CLASSES, unique.incrementAndGet()))
            .toInstance(asList(classes));
    }


    /**
     * Creates a binding to {@code type} that is used as the target of a
     * remote method call with the class's unqualified name as the script name.
     *
     * Note: if you are scoping the result, don't rely on implicit binding.
     * Instead, link the type to itself explicitly. For example,
     * 
     *   bindRemoted(ConcreteService.class)
     *       .to(ConcreteService.class) // this line is required
     *       .in(DwrScopes.SESSION);
     * 
     * This could be considered a bug.
     * @param type the type to bind as a target for remote method calls
     */
    protected  LinkedBindingBuilder bindRemoted(Class type)
    {
        return bind(type)
            .annotatedWith(new RemotedImpl());
    }

    /**
     * Creates a binding to a type that is used as the target of a
     * remote method call with the given {@code scriptName}.
     *
     * Note: if you are scoping the result, don't rely on implicit binding.
     * Instead, link the type to itself explicitly. For example,
     * 
     *   bindRemotedAs("Mixer", ConcreteService.class)
     *       .to(ConcreteService.class) // this line is required
     *       .in(DwrScopes.SESSION);
     * 
     * This could be considered a bug.
     * @param scriptName the name by which the target type will be known to script callers
     * @param type the type to bind as a target for remote method calls
     */
    protected  LinkedBindingBuilder bindRemotedAs(String scriptName, Class type)
    {
        return bind(type)
            .annotatedWith(new RemotedImpl(scriptName));
    }


    /**
     * Creates a binding for a conversion for types with names matching
     * {@code match}.
     * @param match the string describing which types to convert
     */
    protected LinkedBindingBuilder bindConversion(String match)
    {
        return bind(Converter.class)
            .annotatedWith(new ConvertingImpl(match));
    }

    /**
     * Creates a binding for a conversion for {@code type}.
     * @param type the type to be converted
     */
    protected LinkedBindingBuilder bindConversion(Class type)
    {
        return bind(Converter.class)
            .annotatedWith(new ConvertingImpl(type));
    }


    /**
     * Creates a binding for a conversion for {@code type} using an existing
     * conversion for {@code impl}, which must be assignable to {@code type}.
     * The check for an existing conversion happens at run-time.
     * @param type the type to be converted
     * @param impl a type for which a conversion is already defined
     */
    protected  void bindConversion(Class type, Class impl)
    {
        bind(Converter.class)
            .annotatedWith(new ConvertingImpl(type, impl))
            .to(InternalConverter.class); // never used, subverted by InternalConverterManager
    }

    /**
     * Creates a binding for an Ajax filter for the script named
     * {@code scriptName}.
     * @param scriptName the script to filter
     */
    protected LinkedBindingBuilder bindFilter(String scriptName)
    {
        return bind(AjaxFilter.class)
            .annotatedWith(new FilteringImpl(scriptName, unique.incrementAndGet()));
    }

    /**
     * Creates a binding for a global Ajax filter.
     */
    protected LinkedBindingBuilder bindGlobalFilter()
    {
        return bind(AjaxFilter.class)
            .annotatedWith(new FilteringImpl("", unique.incrementAndGet()));
    }


    /**
     * Call this method in
     * {@link org.directwebremoting.guice.AbstractDwrModule#configure configure}
     * to create a binding for a DWR parameter.
     * @param paramName a parameter name supported by DWR
     */
    protected ConstantBindingBuilder bindParameter(ParamName paramName)
    {
        return bindConstant()
            .annotatedWith(new InitParamImpl(paramName));
    }


    /**
     * Used to determine what value {@link #bindDwrScopes() bindDwrScopes()}
     * passes to {@link #bindDwrScopes(boolean) bindDwrScopes(boolean)}.
     * If null, the result of calling {@code !guiceServletModuleExists()}
     * is used.
     */
    volatile Boolean bindPotentiallyConflictingTypes = null;


    private static final AtomicLong unique = new AtomicLong();

    private static final ThreadLocal boundDwrScopes = new ThreadLocal()
    {
        @Override
        protected Boolean initialValue()
        {
            return false;
        }
    };


    private static boolean guiceServletModuleExists()
    {
        try
        {
            LocalUtil.classForName("com.google.inject.servlet.ServletModule");
            return true;
        }
        catch (ClassNotFoundException e)
        {
            return false;
        }
    }
}