freemarker-core/src/main/java/freemarker/core/_JavaVersions.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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 freemarker.core;

import freemarker.log.Logger;
import freemarker.template.Version;
import freemarker.template.utility.SecurityUtilities;

/**
 * Used internally only, might change without notice!
 */
public final class _JavaVersions {
    
    private _JavaVersions() {
        // Not meant to be instantiated
    }

    /**
     * {@code null} if Java 9 is not available, otherwise the object through with the Java 9 operations are available.
     */
    static public final _Java9 JAVA_9 = isAtLeast(9, "java.lang.Module")
            ? tryLoadJavaSupportSingleton(9, _Java9.class)
            : null;

    /**
     * {@code null} if Java 16 is not available, otherwise the object through with the Java 16 operations are available.
     */
    static public final _Java16 JAVA_16 = isAtLeast(16, "java.net.UnixDomainSocketAddress")
            ? tryLoadJavaSupportSingleton(16, _Java16.class)
            : null;

    @SuppressWarnings("unchecked")
    private static <T> T tryLoadJavaSupportSingleton(int javaVersion, Class<T> javaSupportInterface) {
        String implClassName = "freemarker.core._Java" + javaVersion + "Impl";
        try {
            return (T) Class.forName(implClassName)
                    .getField("INSTANCE")
                    .get(null);
        } catch (Exception e) {
            try {
                if (e instanceof ClassNotFoundException) {
                    // Happens when we run JUnit tests
                    Logger.getLogger("freemarker.runtime").warn(
                            "Seems that the Java " + javaVersion + " support class (" + implClassName
                                    + ") wasn't included in the build");
                } else {
                    Logger.getLogger("freemarker.runtime").error(
                            "Failed to load Java " + javaVersion + " support class",
                            e);
                }
            } catch (Exception e2) {
                // Suppressed
            }
            return null;
        }
    }

    private static boolean isAtLeast(int minimumMajorVersion, String proofClassPresence) {
        boolean result = false;
        String vStr = SecurityUtilities.getSystemProperty("java.version", null);
        if (vStr != null) {
            try {
                Version v = new Version(vStr);
                result = v.getMajor() >= minimumMajorVersion;
            } catch (Exception e) {
                // Ignore
            }
        } else {
            try {
                Class.forName(proofClassPresence);
                result = true;
            } catch (Exception e) {
                // Ignore
            }
        }
        return result;
    }

}