/*
 * Copyright 2002-2019 the original author or authors.
 *
 * 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
 *
 *      https://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.springframework.jdbc.datasource;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;

import javax.sql.DataSource;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import org.springframework.core.Constants;
import org.springframework.lang.Nullable;

/**
 * Proxy for a target DataSource, fetching actual JDBC Connections lazily,
 * i.e. not until first creation of a Statement. Connection initialization
 * properties like auto-commit mode, transaction isolation and read-only mode
 * will be kept and applied to the actual JDBC Connection as soon as an
 * actual Connection is fetched (if ever). Consequently, commit and rollback
 * calls will be ignored if no Statements have been created.
 *
 * <p>This DataSource proxy allows to avoid fetching JDBC Connections from
 * a pool unless actually necessary. JDBC transaction control can happen
 * without fetching a Connection from the pool or communicating with the
 * database; this will be done lazily on first creation of a JDBC Statement.
 *
 * <p><b>If you configure both a LazyConnectionDataSourceProxy and a
 * TransactionAwareDataSourceProxy, make sure that the latter is the outermost
 * DataSource.</b> In such a scenario, data access code will talk to the
 * transaction-aware DataSource, which will in turn work with the
 * LazyConnectionDataSourceProxy.
 *
 * <p>Lazy fetching of physical JDBC Connections is particularly beneficial
 * in a generic transaction demarcation environment. It allows you to demarcate
 * transactions on all methods that could potentially perform data access,
 * without paying a performance penalty if no actual data access happens.
 *
 * <p>This DataSource proxy gives you behavior analogous to JTA and a
 * transactional JNDI DataSource (as provided by the Java EE server), even
 * with a local transaction strategy like DataSourceTransactionManager or
 * HibernateTransactionManager. It does not add value with Spring's
 * JtaTransactionManager as transaction strategy.
 *
 * <p>Lazy fetching of JDBC Connections is also recommended for read-only
 * operations with Hibernate, in particular if the chances of resolving the
 * result in the second-level cache are high. This avoids the need to
 * communicate with the database at all for such read-only operations.
 * You will get the same effect with non-transactional reads, but lazy fetching
 * of JDBC Connections allows you to still perform reads in transactions.
 *
 * <p><b>NOTE:</b> This DataSource proxy needs to return wrapped Connections
 * (which implement the {@link ConnectionProxy} interface) in order to handle
 * lazy fetching of an actual JDBC Connection. Use {@link Connection#unwrap}
 * to retrieve the native JDBC Connection.
 *
 * @author Juergen Hoeller
 * @since 1.1.4
 * @see DataSourceTransactionManager
 */
public class LazyConnectionDataSourceProxy extends DelegatingDataSource {

        /** Constants instance for TransactionDefinition. */
        private static final Constants constants = new Constants(Connection.class);

        private static final Log logger = LogFactory.getLog(LazyConnectionDataSourceProxy.class);

        @Nullable
        private Boolean defaultAutoCommit;

        @Nullable
        private Integer defaultTransactionIsolation;


        /**
         * Create a new LazyConnectionDataSourceProxy.
         * @see #setTargetDataSource
         */
        public LazyConnectionDataSourceProxy() {
        }

        /**
         * Create a new LazyConnectionDataSourceProxy.
         * @param targetDataSource the target DataSource
         */
        public LazyConnectionDataSourceProxy(DataSource targetDataSource) {
                setTargetDataSource(targetDataSource);
                afterPropertiesSet();
        }


        /**
         * Set the default auto-commit mode to expose when no target Connection
         * has been fetched yet (-> actual JDBC Connection default not known yet).
         * <p>If not specified, the default gets determined by checking a target
         * Connection on startup. If that check fails, the default will be determined
         * lazily on first access of a Connection.
         * @see java.sql.Connection#setAutoCommit
         */
        public void setDefaultAutoCommit(boolean defaultAutoCommit) {
                this.defaultAutoCommit = defaultAutoCommit;
        }

        /**
         * Set the default transaction isolation level to expose when no target Connection
         * has been fetched yet (-> actual JDBC Connection default not known yet).
         * <p>This property accepts the int constant value (e.g. 8) as defined in the
         * {@link java.sql.Connection} interface; it is mainly intended for programmatic
         * use. Consider using the "defaultTransactionIsolationName" property for setting
         * the value by name (e.g. "TRANSACTION_SERIALIZABLE").
         * <p>If not specified, the default gets determined by checking a target
         * Connection on startup. If that check fails, the default will be determined
         * lazily on first access of a Connection.
         * @see #setDefaultTransactionIsolationName
         * @see java.sql.Connection#setTransactionIsolation
         */
        public void setDefaultTransactionIsolation(int defaultTransactionIsolation) {
                this.defaultTransactionIsolation = defaultTransactionIsolation;
        }

        /**
         * Set the default transaction isolation level by the name of the corresponding
         * constant in {@link java.sql.Connection}, e.g. "TRANSACTION_SERIALIZABLE".
         * @param constantName name of the constant
         * @see #setDefaultTransactionIsolation
         * @see java.sql.Connection#TRANSACTION_READ_UNCOMMITTED
         * @see java.sql.Connection#TRANSACTION_READ_COMMITTED
         * @see java.sql.Connection#TRANSACTION_REPEATABLE_READ
         * @see java.sql.Connection#TRANSACTION_SERIALIZABLE
         */
        public void setDefaultTransactionIsolationName(String constantName) {
                setDefaultTransactionIsolation(constants.asNumber(constantName).intValue());
        }


        @Override
        public void afterPropertiesSet() {
                super.afterPropertiesSet();

                // Determine default auto-commit and transaction isolation
                // via a Connection from the target DataSource, if possible.
                if (this.defaultAutoCommit == null || this.defaultTransactionIsolation == null) {
                        try {
                                try (Connection con = obtainTargetDataSource().getConnection()) {
                                        checkDefaultConnectionProperties(con);
                                }
                        }
                        catch (SQLException ex) {
                                logger.debug("Could not retrieve default auto-commit and transaction isolation settings", ex);
                        }
                }
        }

        /**
         * Check the default connection properties (auto-commit, transaction isolation),
         * keeping them to be able to expose them correctly without fetching an actual
         * JDBC Connection from the target DataSource.
         * <p>This will be invoked once on startup, but also for each retrieval of a
         * target Connection. If the check failed on startup (because the database was
         * down), we'll lazily retrieve those settings.
         * @param con the Connection to use for checking
         * @throws SQLException if thrown by Connection methods
         */
        protected synchronized void checkDefaultConnectionProperties(Connection con) throws SQLException {
                if (this.defaultAutoCommit == null) {
                        this.defaultAutoCommit = con.getAutoCommit();
                }
                if (this.defaultTransactionIsolation == null) {
                        this.defaultTransactionIsolation = con.getTransactionIsolation();
                }
        }

        /**
         * Expose the default auto-commit value.
         */
        @Nullable
        protected Boolean defaultAutoCommit() {
                return this.defaultAutoCommit;
        }

        /**
         * Expose the default transaction isolation value.
         */
        @Nullable
        protected Integer defaultTransactionIsolation() {
                return this.defaultTransactionIsolation;
        }


        /**
         * Return a Connection handle that lazily fetches an actual JDBC Connection
         * when asked for a Statement (or PreparedStatement or CallableStatement).
         * <p>The returned Connection handle implements the ConnectionProxy interface,
         * allowing to retrieve the underlying target Connection.
         * @return a lazy Connection handle
         * @see ConnectionProxy#getTargetConnection()
         */
        @Override
        public Connection getConnection() throws SQLException {
                return (Connection) Proxy.newProxyInstance(
                                ConnectionProxy.class.getClassLoader(),
                                new Class<?>[] {ConnectionProxy.class},
                                new LazyConnectionInvocationHandler());
        }

        /**
         * Return a Connection handle that lazily fetches an actual JDBC Connection
         * when asked for a Statement (or PreparedStatement or CallableStatement).
         * <p>The returned Connection handle implements the ConnectionProxy interface,
         * allowing to retrieve the underlying target Connection.
         * @param username the per-Connection username
         * @param password the per-Connection password
         * @return a lazy Connection handle
         * @see ConnectionProxy#getTargetConnection()
         */
        @Override
        public Connection getConnection(String username, String password) throws SQLException {
                return (Connection) Proxy.newProxyInstance(
                                ConnectionProxy.class.getClassLoader(),
                                new Class<?>[] {ConnectionProxy.class},
                                new LazyConnectionInvocationHandler(username, password));
        }


        /**
         * Invocation handler that defers fetching an actual JDBC Connection
         * until first creation of a Statement.
         */
        private class LazyConnectionInvocationHandler implements InvocationHandler {

                @Nullable
                private String username;

                @Nullable
                private String password;

                @Nullable
                private Boolean autoCommit;

                @Nullable
                private Integer transactionIsolation;

                private boolean readOnly = false;

                private int holdability = ResultSet.CLOSE_CURSORS_AT_COMMIT;

                private boolean closed = false;

                @Nullable
                private Connection target;

                public LazyConnectionInvocationHandler() {
                        this.autoCommit = defaultAutoCommit();
                        this.transactionIsolation = defaultTransactionIsolation();
                }

                public LazyConnectionInvocationHandler(String username, String password) {
                        this();
                        this.username = username;
                        this.password = password;
                }

                @Override
                @Nullable
                public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
                        // Invocation on ConnectionProxy interface coming in...

                        if (method.getName().equals("equals")) {
                                // We must avoid fetching a target Connection for "equals".
                                // Only consider equal when proxies are identical.
                                return (proxy == args[0]);
                        }
                        else if (method.getName().equals("hashCode")) {
                                // We must avoid fetching a target Connection for "hashCode",
                                // and we must return the same hash code even when the target
                                // Connection has been fetched: use hashCode of Connection proxy.
                                return System.identityHashCode(proxy);
                        }
                        else if (method.getName().equals("unwrap")) {
                                if (((Class<?>) args[0]).isInstance(proxy)) {
                                        return proxy;
                                }
                        }
                        else if (method.getName().equals("isWrapperFor")) {
                                if (((Class<?>) args[0]).isInstance(proxy)) {
                                        return true;
                                }
                        }
                        else if (method.getName().equals("getTargetConnection")) {
                                // Handle getTargetConnection method: return underlying connection.
                                return getTargetConnection(method);
                        }

                        if (!hasTargetConnection()) {
                                // No physical target Connection kept yet ->
                                // resolve transaction demarcation methods without fetching
                                // a physical JDBC Connection until absolutely necessary.

                                if (method.getName().equals("toString")) {
                                        return "Lazy Connection proxy for target DataSource [" + getTargetDataSource() + "]";
                                }
                                else if (method.getName().equals("getAutoCommit")) {
                                        if (this.autoCommit != null) {
                                                return this.autoCommit;
                                        }
                                        // Else fetch actual Connection and check there,
                                        // because we didn't have a default specified.
                                }
                                else if (method.getName().equals("setAutoCommit")) {
                                        this.autoCommit = (Boolean) args[0];
                                        return null;
                                }
                                else if (method.getName().equals("getTransactionIsolation")) {
                                        if (this.transactionIsolation != null) {
                                                return this.transactionIsolation;
                                        }
                                        // Else fetch actual Connection and check there,
                                        // because we didn't have a default specified.
                                }
                                else if (method.getName().equals("setTransactionIsolation")) {
                                        this.transactionIsolation = (Integer) args[0];
                                        return null;
                                }
                                else if (method.getName().equals("isReadOnly")) {
                                        return this.readOnly;
                                }
                                else if (method.getName().equals("setReadOnly")) {
                                        this.readOnly = (Boolean) args[0];
                                        return null;
                                }
                                else if (method.getName().equals("getHoldability")) {
                                        return this.holdability;
                                }
                                else if (method.getName().equals("setHoldability")) {
                                        this.holdability = (Integer) args[0];
                                        return null;
                                }
                                else if (method.getName().equals("commit") || method.getName().equals("rollback")) {
                                        // Ignore: no statements created yet.
                                        return null;
                                }
                                else if (method.getName().equals("getWarnings") || method.getName().equals("clearWarnings")) {
                                        // Ignore: no warnings to expose yet.
                                        return null;
                                }
                                else if (method.getName().equals("close")) {
                                        // Ignore: no target connection yet.
                                        this.closed = true;
                                        return null;
                                }
                                else if (method.getName().equals("isClosed")) {
                                        return this.closed;
                                }
                                else if (this.closed) {
                                        // Connection proxy closed, without ever having fetched a
                                        // physical JDBC Connection: throw corresponding SQLException.
                                        throw new SQLException("Illegal operation: connection is closed");
                                }
                        }

                        // Target Connection already fetched,
                        // or target Connection necessary for current operation ->
                        // invoke method on target connection.
                        try {
                                return method.invoke(getTargetConnection(method), args);
                        }
                        catch (InvocationTargetException ex) {
                                throw ex.getTargetException();
                        }
                }

                /**
                 * Return whether the proxy currently holds a target Connection.
                 */
                private boolean hasTargetConnection() {
                        return (this.target != null);
                }

                /**
                 * Return the target Connection, fetching it and initializing it if necessary.
                 */
                private Connection getTargetConnection(Method operation) throws SQLException {
                        if (this.target == null) {
                                // No target Connection held -> fetch one.
                                if (logger.isTraceEnabled()) {
                                        logger.trace("Connecting to database for operation '" + operation.getName() + "'");
                                }

                                // Fetch physical Connection from DataSource.
                                this.target = (this.username != null) ?
                                                obtainTargetDataSource().getConnection(this.username, this.password) :
                                                obtainTargetDataSource().getConnection();

                                // If we still lack default connection properties, check them now.
                                checkDefaultConnectionProperties(this.target);

                                // Apply kept transaction settings, if any.
                                if (this.readOnly) {
                                        try {
                                                this.target.setReadOnly(true);
                                        }
                                        catch (Exception ex) {
                                                // "read-only not supported" -> ignore, it's just a hint anyway
                                                logger.debug("Could not set JDBC Connection read-only", ex);
                                        }
                                }
                                if (this.transactionIsolation != null &&
                                                !this.transactionIsolation.equals(defaultTransactionIsolation())) {
                                        this.target.setTransactionIsolation(this.transactionIsolation);
                                }
                                if (this.autoCommit != null && this.autoCommit != this.target.getAutoCommit()) {
                                        this.target.setAutoCommit(this.autoCommit);
                                }
                        }

                        else {
                                // Target Connection already held -> return it.
                                if (logger.isTraceEnabled()) {
                                        logger.trace("Using existing database connection for operation '" + operation.getName() + "'");
                                }
                        }

                        return this.target;
                }
        }

}