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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
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 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
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package jdk.jshell.execution;

import java.io.PrintWriter;
import java.io.StringWriter;
import java.util.HashMap;
import java.util.Map;
import java.util.logging.Level;
import java.util.logging.Logger;
import jdk.jshell.spi.ExecutionControl;
import jdk.jshell.spi.ExecutionControlProvider;
import jdk.jshell.spi.ExecutionEnv;

Tries other providers in sequence until one works.
Since:
9/**
 * Tries other providers in sequence until one works.
 *
 * @since 9
 */
public class FailOverExecutionControlProvider  implements ExecutionControlProvider{

    private Logger logger = null;

    
Create an instance. The instance can be used to start and return an ExecutionControl instance by attempting to start a series of ExecutionControl specs, until one is successful. 
See Also:
generate(ExecutionEnv, Map<String,String>)/**
     * Create an instance. The instance can be used to start and return an
     * {@link ExecutionControl} instance by attempting to start a series of
     * {@code ExecutionControl} specs, until one is successful.
     *
     * @see #generate(jdk.jshell.spi.ExecutionEnv, java.util.Map)
     */
    public FailOverExecutionControlProvider() {
    }

    
The unique name of this ExecutionControlProvider. 
Returns:
"failover"/**
     * The unique name of this {@code ExecutionControlProvider}.
     *
     * @return "failover"
     */
    @Override
    public String name() {
        return "failover";
    }

    
Create and return the default parameter map for this ExecutionControlProvider. There are ten parameters, "0" through "9", their values are ExecutionControlProvider specification strings, or empty string. 
Returns:
a default parameter map/**
     * Create and return the default parameter map for this
     * {@code ExecutionControlProvider}. There are ten parameters, "0" through
     * "9", their values are {@code ExecutionControlProvider} specification
     * strings, or empty string.
     *
     * @return a default parameter map
     */
    @Override
    public Map<String, String> defaultParameters() {
        Map<String, String> dp = new HashMap<>();
        dp.put("0", "jdi");
        for (int i = 1; i <= 9; ++i) {
            dp.put("" + i, "");
        }
        return dp;
    }

    
Create and return a locally executing ExecutionControl instance. At least one parameter should have a spec. 
Params:
env – the execution environment, provided by JShell
parameters – the modified parameter map.
Throws:
Throwable – if all the given providers fail, the exception that
occurred on the first attempt to create the execution engine.
Returns:
the execution engine/**
     * Create and return a locally executing {@code ExecutionControl} instance.
     * At least one parameter should have a spec.
     *
     * @param env the execution environment, provided by JShell
     * @param parameters the modified parameter map.
     * @return the execution engine
     * @throws Throwable if all the given providers fail, the exception that
     * occurred on the first attempt to create the execution engine.
     */
    @Override
    public ExecutionControl generate(ExecutionEnv env, Map<String, String> parameters)
            throws Throwable {
        Throwable thrown = null;
        for (int i = 0; i <= 9; ++i) {
            String param = parameters.get("" + i);
            if (param != null && !param.isEmpty()) {
                try {
                    ExecutionControl ec =  ExecutionControl.generate(env, param);
                    logger().finest(
                            String.format("FailOverExecutionControlProvider: Success %s -- %d = %s\n",
                                    name(), i, param));
                    return ec;
                } catch (Throwable ex) {
                    logger().warning(
                            String.format("FailOverExecutionControlProvider: Failure %s -- %d = %s -- %s\n",
                                    name(), i, param, ex.toString()));
                    StringWriter writer = new StringWriter();
                    PrintWriter log = new PrintWriter(writer);
                    log.println("FailOverExecutionControlProvider:");
                    ex.printStackTrace(log);
                    log.flush();
                    logger().fine(writer.toString());
                    // only care about the first, and only if they all fail
                    if (thrown == null) {
                        thrown = ex;
                    }
                }
            }

        }
        logger().severe("FailOverExecutionControlProvider: Terminating, failovers exhausted");
        if (thrown == null) {
            throw new IllegalArgumentException("All least one parameter must be set to a provider.");
        }
        throw thrown;
    }

    private Logger logger() {
        if (logger == null) {
            logger = Logger.getLogger("jdk.jshell.execution");
            if (logger.getLevel() == null) {
                // Logging has not been specifically requested, turn it off
                logger.setLevel(Level.OFF);
            }
        }
        return logger;
    }

}