org.codehaus.plexus.util.introspection.ReflectionValueExtractor Maven / Gradle / Ivy

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package org.codehaus.plexus.util.introspection;

/*
 * Copyright The Codehaus Foundation.
 *
 * 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.
 */

import java.lang.ref.WeakReference;
import java.lang.reflect.Array;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.List;
import java.util.Map;
import java.util.WeakHashMap;

import org.codehaus.plexus.util.StringUtils;

/**
 * 
 * Using simple dotted expressions to extract the values from an Object instance, For example we might want to extract a
 * value like: project.build.sourceDirectory
 * 
 * 
 * The implementation supports indexed, nested and mapped properties similar to the JSP way.
 * 
 * 
 * @author Jason van Zyl 
 * @author Vincent Siveton
 *
 * @see http://struts.apache.org/1.x/struts-taglib/indexedprops.html
 */
public class ReflectionValueExtractor
{
    private static final Class[] CLASS_ARGS = new Class[0];

    private static final Object[] OBJECT_ARGS = new Object[0];

    /**
     * Use a WeakHashMap here, so the keys (Class objects) can be garbage collected. This approach prevents permgen
     * space overflows due to retention of discarded classloaders.
     */
    private static final Map, WeakReference> classMaps =
        new WeakHashMap, WeakReference>();

    static final int EOF = -1;

    static final char PROPERTY_START = '.';

    static final char INDEXED_START = '[';

    static final char INDEXED_END = ']';

    static final char MAPPED_START = '(';

    static final char MAPPED_END = ')';

    static class Tokenizer
    {
        final String expression;

        int idx;

        public Tokenizer( String expression )
        {
            this.expression = expression;
        }

        public int peekChar()
        {
            return idx < expression.length() ? expression.charAt( idx ) : EOF;
        }

        public int skipChar()
        {
            return idx < expression.length() ? expression.charAt( idx++ ) : EOF;
        }

        public String nextToken( char delimiter )
        {
            int start = idx;

            while ( idx < expression.length() && delimiter != expression.charAt( idx ) )
            {
                idx++;
            }

            // delimiter MUST be present
            if ( idx <= start || idx >= expression.length() )
            {
                return null;
            }

            return expression.substring( start, idx++ );
        }

        public String nextPropertyName()
        {
            final int start = idx;

            while ( idx < expression.length() && Character.isJavaIdentifierPart( expression.charAt( idx ) ) )
            {
                idx++;
            }

            // property name does not require delimiter
            if ( idx <= start || idx > expression.length() )
            {
                return null;
            }

            return expression.substring( start, idx );
        }

        public int getPosition()
        {
            return idx < expression.length() ? idx : EOF;
        }

        // to make tokenizer look pretty in debugger
        @Override
        public String toString()
        {
            return idx < expression.length() ? expression.substring( idx ) : "";
        }
    }

    private ReflectionValueExtractor()
    {
    }

    /**
     * The implementation supports indexed, nested and mapped properties.
     * 
     * 
     * nested properties should be defined by a dot, i.e. "user.address.street"
     * indexed properties (java.util.List or array instance) should be contains (\\w+)\\[(\\d+)\\]
     * pattern, i.e. "user.addresses[1].street"
     * mapped properties should be contains (\\w+)\\((.+)\\) pattern, i.e.
     * "user.addresses(myAddress).street"
     * 
     * 
     * @param expression not null expression
     * @param root not null object
     * @return the object defined by the expression
     * @throws Exception if any
     */
    public static Object evaluate( String expression, Object root )
        throws Exception
    {
        return evaluate( expression, root, true );
    }

    /**
     * The implementation supports indexed, nested and mapped properties.
     * 
     * 
     * nested properties should be defined by a dot, i.e. "user.address.street"
     * indexed properties (java.util.List or array instance) should be contains (\\w+)\\[(\\d+)\\]
     * pattern, i.e. "user.addresses[1].street"
     * mapped properties should be contains (\\w+)\\((.+)\\) pattern, i.e.
     * "user.addresses(myAddress).street"
     * 
     * 
     * @param expression not null expression
     * @param root not null object
     * @param trimRootToken  root start
     * @return the object defined by the expression
     * @throws Exception if any
     */
    // TODO: don't throw Exception
    public static Object evaluate( String expression, final Object root, final boolean trimRootToken )
        throws Exception
    {
        Object value = root;

        // ----------------------------------------------------------------------
        // Walk the dots and retrieve the ultimate value desired from the
        // MavenProject instance.
        // ----------------------------------------------------------------------

        if ( StringUtils.isEmpty( expression ) || !Character.isJavaIdentifierStart( expression.charAt( 0 ) ) )
        {
            return null;
        }

        boolean hasDots = expression.indexOf( PROPERTY_START ) >= 0;

        final Tokenizer tokenizer;
        if ( trimRootToken && hasDots )
        {
            tokenizer = new Tokenizer( expression );
            tokenizer.nextPropertyName();
            if ( tokenizer.getPosition() == EOF )
            {
                return null;
            }
        }
        else
        {
            tokenizer = new Tokenizer( "." + expression );
        }

        int propertyPosition = tokenizer.getPosition();
        while ( value != null && tokenizer.peekChar() != EOF )
        {
            switch ( tokenizer.skipChar() )
            {
                case INDEXED_START:
                    value = getIndexedValue( expression, propertyPosition, tokenizer.getPosition(), value,
                                             tokenizer.nextToken( INDEXED_END ) );
                    break;
                case MAPPED_START:
                    value = getMappedValue( expression, propertyPosition, tokenizer.getPosition(), value,
                                            tokenizer.nextToken( MAPPED_END ) );
                    break;
                case PROPERTY_START:
                    propertyPosition = tokenizer.getPosition();
                    value = getPropertyValue( value, tokenizer.nextPropertyName() );
                    break;
                default:
                    // could not parse expression
                    return null;
            }
        }

        return value;
    }

    private static Object getMappedValue( final String expression, final int from, final int to, final Object value,
                                          final String key )
        throws Exception
    {
        if ( value == null || key == null )
        {
            return null;
        }

        if ( value instanceof Map )
        {
            Object[] localParams = new Object[] { key };
            ClassMap classMap = getClassMap( value.getClass() );
            Method method = classMap.findMethod( "get", localParams );
            return method.invoke( value, localParams );
        }

        final String message =
            String.format( "The token '%s' at position '%d' refers to a java.util.Map, but the value seems is an instance of '%s'",
                           expression.subSequence( from, to ), from, value.getClass() );

        throw new Exception( message );
    }

    private static Object getIndexedValue( final String expression, final int from, final int to, final Object value,
                                           final String indexStr )
        throws Exception
    {
        try
        {
            int index = Integer.parseInt( indexStr );

            if ( value.getClass().isArray() )
            {
                return Array.get( value, index );
            }

            if ( value instanceof List )
            {
                ClassMap classMap = getClassMap( value.getClass() );
                // use get method on List interface
                Object[] localParams = new Object[] { index };
                Method method = classMap.findMethod( "get", localParams );
                return method.invoke( value, localParams );
            }
        }
        catch ( NumberFormatException e )
        {
            return null;
        }
        catch ( InvocationTargetException e )
        {
            // catch array index issues gracefully, otherwise release
            if ( e.getCause() instanceof IndexOutOfBoundsException )
            {
                return null;
            }

            throw e;
        }

        final String message =
            String.format( "The token '%s' at position '%d' refers to a java.util.List or an array, but the value seems is an instance of '%s'",
                           expression.subSequence( from, to ), from, value.getClass() );

        throw new Exception( message );
    }

    private static Object getPropertyValue( Object value, String property )
        throws Exception
    {
        if ( value == null || property == null )
        {
            return null;
        }

        ClassMap classMap = getClassMap( value.getClass() );
        String methodBase = StringUtils.capitalizeFirstLetter( property );
        String methodName = "get" + methodBase;
        Method method = classMap.findMethod( methodName, CLASS_ARGS );

        if ( method == null )
        {
            // perhaps this is a boolean property??
            methodName = "is" + methodBase;

            method = classMap.findMethod( methodName, CLASS_ARGS );
        }

        if ( method == null )
        {
            return null;
        }

        try
        {
            return method.invoke( value, OBJECT_ARGS );
        }
        catch ( InvocationTargetException e )
        {
            throw e;
        }
    }

    private static ClassMap getClassMap( Class clazz )
    {

        WeakReference softRef = classMaps.get( clazz );

        ClassMap classMap;

        if ( softRef == null || ( classMap = softRef.get() ) == null )
        {
            classMap = new ClassMap( clazz );

            classMaps.put( clazz, new WeakReference( classMap ) );
        }

        return classMap;
    }
}