聊聊spring data jpa的JpaQueryLookupStrategy

public interface QueryLookupStrategy {
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    public static enum Key {
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        CREATE, USE_DECLARED_QUERY, CREATE_IF_NOT_FOUND;
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        /**
         * Returns a strategy key from the given XML value.
         *
         * @param xml
         * @return a strategy key from the given XML value
         */
        @Nullable
        public static Key create(String xml) {
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            if (!StringUtils.hasText(xml)) {
                return null;
            }
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            return valueOf(xml.toUpperCase(Locale.US).replace("-", "_"));
        }
    }
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    /**
     * Resolves a {@link RepositoryQuery} from the given {@link QueryMethod} that can be executed afterwards.
     *
     * @param method will never be {@literal null}.
     * @param metadata will never be {@literal null}.
     * @param factory will never be {@literal null}.
     * @param namedQueries will never be {@literal null}.
     * @return
     */
    RepositoryQuery resolveQuery(Method method, RepositoryMetadata metadata, ProjectionFactory factory,
            NamedQueries namedQueries);
}

public interface RepositoryQuery {
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    /**
     * Executes the {@link RepositoryQuery} with the given parameters.
     *
     * @param parameters must not be {@literal null}.
     * @return execution result. Can be {@literal null}.
     */
    @Nullable
    Object execute(Object[] parameters);
​
    /**
     * Returns the related {@link QueryMethod}.
     *
     * @return never {@literal null}.
     */
    QueryMethod getQueryMethod();
}

    /**
     * Base class for {@link QueryLookupStrategy} implementations that need access to an {@link EntityManager}.
     *
     * @author Oliver Gierke
     * @author Thomas Darimont
     */
    private abstract static class AbstractQueryLookupStrategy implements QueryLookupStrategy {
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        private final EntityManager em;
        private final QueryExtractor provider;
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        /**
         * Creates a new {@link AbstractQueryLookupStrategy}.
         *
         * @param em
         * @param extractor
         */
        public AbstractQueryLookupStrategy(EntityManager em, QueryExtractor extractor) {
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            this.em = em;
            this.provider = extractor;
        }
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        /*
         * (non-Javadoc)
         * @see org.springframework.data.repository.query.QueryLookupStrategy#resolveQuery(java.lang.reflect.Method, org.springframework.data.repository.core.RepositoryMetadata, org.springframework.data.projection.ProjectionFactory, org.springframework.data.repository.core.NamedQueries)
         */
        @Override
        public final RepositoryQuery resolveQuery(Method method, RepositoryMetadata metadata, ProjectionFactory factory,
                NamedQueries namedQueries) {
            return resolveQuery(new JpaQueryMethod(method, metadata, factory, provider), em, namedQueries);
        }
​
        protected abstract RepositoryQuery resolveQuery(JpaQueryMethod method, EntityManager em, NamedQueries namedQueries);
    }

    /**
     * {@link QueryLookupStrategy} to create a query from the method name.
     *
     * @author Oliver Gierke
     * @author Thomas Darimont
     */
    private static class CreateQueryLookupStrategy extends AbstractQueryLookupStrategy {
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        private final PersistenceProvider persistenceProvider;
        private final EscapeCharacter escape;
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        public CreateQueryLookupStrategy(EntityManager em, QueryExtractor extractor, EscapeCharacter escape) {
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            super(em, extractor);
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            this.persistenceProvider = PersistenceProvider.fromEntityManager(em);
            this.escape = escape;
        }
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        @Override
        protected RepositoryQuery resolveQuery(JpaQueryMethod method, EntityManager em, NamedQueries namedQueries) {
            return new PartTreeJpaQuery(method, em, persistenceProvider, escape);
        }
    }

    /**
     * {@link QueryLookupStrategy} that tries to detect a declared query declared via {@link Query} annotation followed by
     * a JPA named query lookup.
     *
     * @author Oliver Gierke
     * @author Thomas Darimont
     */
    private static class DeclaredQueryLookupStrategy extends AbstractQueryLookupStrategy {
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        private final QueryMethodEvaluationContextProvider evaluationContextProvider;
​
        /**
         * Creates a new {@link DeclaredQueryLookupStrategy}.
         *
         * @param em
         * @param extractor
         * @param evaluationContextProvider
         */
        public DeclaredQueryLookupStrategy(EntityManager em, QueryExtractor extractor,
                QueryMethodEvaluationContextProvider evaluationContextProvider) {
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            super(em, extractor);
            this.evaluationContextProvider = evaluationContextProvider;
        }
​
        /*
         * (non-Javadoc)
         * @see org.springframework.data.jpa.repository.query.JpaQueryLookupStrategy.AbstractQueryLookupStrategy#resolveQuery(org.springframework.data.jpa.repository.query.JpaQueryMethod, javax.persistence.EntityManager, org.springframework.data.repository.core.NamedQueries)
         */
        @Override
        protected RepositoryQuery resolveQuery(JpaQueryMethod method, EntityManager em, NamedQueries namedQueries) {
​
            RepositoryQuery query = JpaQueryFactory.INSTANCE.fromQueryAnnotation(method, em, evaluationContextProvider);
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            if (null != query) {
                return query;
            }
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            query = JpaQueryFactory.INSTANCE.fromProcedureAnnotation(method, em);
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            if (null != query) {
                return query;
            }
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            String name = method.getNamedQueryName();
            if (namedQueries.hasQuery(name)) {
                return JpaQueryFactory.INSTANCE.fromMethodWithQueryString(method, em, namedQueries.getQuery(name),
                        evaluationContextProvider);
            }
​
            query = NamedQuery.lookupFrom(method, em);
​
            if (null != query) {
                return query;
            }
​
            throw new IllegalStateException(
                    String.format("Did neither find a NamedQuery nor an annotated query for method %s!", method));
        }
    }

    /**
     * {@link QueryLookupStrategy} to try to detect a declared query first (
     * {@link org.springframework.data.jpa.repository.Query}, JPA named query). In case none is found we fall back on
     * query creation.
     *
     * @author Oliver Gierke
     * @author Thomas Darimont
     */
    private static class CreateIfNotFoundQueryLookupStrategy extends AbstractQueryLookupStrategy {
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        private final DeclaredQueryLookupStrategy lookupStrategy;
        private final CreateQueryLookupStrategy createStrategy;
​
        /**
         * Creates a new {@link CreateIfNotFoundQueryLookupStrategy}.
         *
         * @param em
         * @param extractor
         * @param createStrategy
         * @param lookupStrategy
         */
        public CreateIfNotFoundQueryLookupStrategy(EntityManager em, QueryExtractor extractor,
                CreateQueryLookupStrategy createStrategy, DeclaredQueryLookupStrategy lookupStrategy) {
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            super(em, extractor);
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            this.createStrategy = createStrategy;
            this.lookupStrategy = lookupStrategy;
        }
​
        /*
         * (non-Javadoc)
         * @see org.springframework.data.jpa.repository.query.JpaQueryLookupStrategy.AbstractQueryLookupStrategy#resolveQuery(org.springframework.data.jpa.repository.query.JpaQueryMethod, javax.persistence.EntityManager, org.springframework.data.repository.core.NamedQueries)
         */
        @Override
        protected RepositoryQuery resolveQuery(JpaQueryMethod method, EntityManager em, NamedQueries namedQueries) {
​
            try {
                return lookupStrategy.resolveQuery(method, em, namedQueries);
            } catch (IllegalStateException e) {
                return createStrategy.resolveQuery(method, em, namedQueries);
            }
        }
    }

public final class JpaQueryLookupStrategy {
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    /**
     * Private constructor to prevent instantiation.
     */
    private JpaQueryLookupStrategy() {}
​
    //......
​
    /**
     * Creates a {@link QueryLookupStrategy} for the given {@link EntityManager} and {@link Key}.
     *
     * @param em must not be {@literal null}.
     * @param key may be {@literal null}.
     * @param extractor must not be {@literal null}.
     * @param evaluationContextProvider must not be {@literal null}.
     * @param escape
     * @return
     */
    public static QueryLookupStrategy create(EntityManager em, @Nullable Key key, QueryExtractor extractor,
            QueryMethodEvaluationContextProvider evaluationContextProvider, EscapeCharacter escape) {
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        Assert.notNull(em, "EntityManager must not be null!");
        Assert.notNull(extractor, "QueryExtractor must not be null!");
        Assert.notNull(evaluationContextProvider, "EvaluationContextProvider must not be null!");
​
        switch (key != null ? key : Key.CREATE_IF_NOT_FOUND) {
            case CREATE:
                return new CreateQueryLookupStrategy(em, extractor, escape);
            case USE_DECLARED_QUERY:
                return new DeclaredQueryLookupStrategy(em, extractor, evaluationContextProvider);
            case CREATE_IF_NOT_FOUND:
                return new CreateIfNotFoundQueryLookupStrategy(em, extractor,
                        new CreateQueryLookupStrategy(em, extractor, escape),
                        new DeclaredQueryLookupStrategy(em, extractor, evaluationContextProvider));
            default:
                throw new IllegalArgumentException(String.format("Unsupported query lookup strategy %s!", key));
        }
    }
}