Introduce Java version checker

Today when users start Elasticsearch with their Java configuration
pointing to a pre-Java 8 install, they encounter a cryptic message:

    Exception in thread "main" java.lang.UnsupportedClassVersionError:
    org/elasticsearch/bootstrap/Elasticsearch : Unsupported major.minor
    version 52.0

They often think that they have Java 8 installed but if their JAVA_HOME
or other configuration is causing them to start with a pre-Java 8
install, this error message does not help them.

We introduce a Java version checker that runs on Java 6 as part of the
startup scripts. If the Java version is pre-Java 8, we can display a
helpful error message to the user informing them of the Java version
that the runtime was started with. Otherwise, Elasticsearch starts as it
does today.

distribution/src/main/resources/bin/elasticsearch

@@ -143,6 +143,13 @@ if [ "x$JAVA_OPTS" != "x" ]; then
     echo "Please pass JVM parameters via ES_JAVA_OPTS instead"
 fi
 
+"$JAVA" -cp "$ES_CLASSPATH" org.elasticsearch.tools.JavaVersionChecker
+
+if [ $? -ne 0 ]; then
+    echo "Elasticsearch requires at least Java 8 but your Java version from $JAVA does not meet this requirement"
+    exit 1
+fi
+
 # full hostname passed through cut for portability on systems that do not support hostname -s
 # export on separate line for shells that do not support combining definition and export
 HOSTNAME=`hostname | cut -d. -f1`

distribution/src/main/resources/bin/elasticsearch.in.bat

@@ -22,4 +22,12 @@ ECHO additional elements via the plugin mechanism, or if code must really be 1>&
 ECHO added to the main classpath, add jars to lib\, unsupported 1>&2
 EXIT /B 1
 )
+
+%JAVA% -cp "%ES_CLASSPATH%" "org.elasticsearch.tools.JavaVersionChecker"
+
+IF ERRORLEVEL 1 (
+    ECHO Elasticsearch requires at least Java 8 but your Java version from %JAVA% does not meet this requirement
+    EXIT /B 1
+)
+
 set ES_PARAMS=-Delasticsearch -Des.path.home="%ES_HOME%"

distribution/tools/java-version-checker/build.gradle

@@ -17,12 +17,25 @@
  * under the License.
  */
 
+
+import org.elasticsearch.gradle.precommit.PrecommitTasks
 import org.gradle.api.JavaVersion
 
 apply plugin: 'elasticsearch.build'
+apply plugin: 'ru.vyarus.animalsniffer'
 
-targetCompatibility = JavaVersion.VERSION_1_6
 sourceCompatibility = JavaVersion.VERSION_1_6
+targetCompatibility = JavaVersion.VERSION_1_6
+
+dependencies {
+    signature "org.codehaus.mojo.signature:java16:1.0@signature"
+}
+
+forbiddenApisMain {
+    // java-version-checker does not depend on core so only JDK signatures should be checked
+    signaturesURLs = [PrecommitTasks.getResource('/forbidden/jdk-signatures.txt')]
+}
 
 test.enabled = false
 loggerUsageCheck.enabled = false
+jarHell.enabled=false

distribution/tools/java-version-checker/src/main/java/org/elasticsearch/tools/JavaVersionChecker.java

@@ -0,0 +1,88 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch 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 org.elasticsearch.tools;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+/**
+ * Simple program that checks if the runtime Java version is at least 1.8.
+ */
+final class JavaVersionChecker {
+
+    private JavaVersionChecker() {
+    }
+
+    private static final List<Integer> JAVA_8 = Arrays.asList(1, 8);
+
+    /**
+     * The main entry point. The exit code is 0 if the Java version is at least 1.8, otherwise the exit code is 1.
+     *
+     * @param args the args to the program which are rejected if not empty
+     */
+    public static void main(final String[] args) {
+        // no leniency!
+        if (args.length != 0) {
+            throw new IllegalArgumentException("expected zero arguments but was: " + Arrays.toString(args));
+        }
+        final String javaSpecificationVersion = System.getProperty("java.specification.version");
+        final List<Integer> current = parse(javaSpecificationVersion);
+        if (compare(current, JAVA_8) < 0) {
+            exit(1);
+        }
+        exit(0);
+    }
+
+    private static List<Integer> parse(final String value) {
+        if (!value.matches("^0*[0-9]+(\\.[0-9]+)*$")) {
+            throw new IllegalArgumentException(value);
+        }
+
+        final List<Integer> version = new ArrayList<Integer>();
+        final String[] components = value.split("\\.");
+        for (final String component : components) {
+            version.add(Integer.valueOf(component));
+        }
+        return version;
+    }
+
+    private static int compare(final List<Integer> left, final List<Integer> right) {
+        // lexicographically compare two lists, treating missing entries as zeros
+        final int len = Math.max(left.size(), right.size());
+        for (int i = 0; i < len; i++) {
+            final int l = (i < left.size()) ? left.get(i) : 0;
+            final int r = (i < right.size()) ? right.get(i) : 0;
+            if (l < r) {
+                return -1;
+            }
+            if (r < l) {
+                return 1;
+            }
+        }
+        return 0;
+    }
+
+    @SuppressForbidden(reason = "exit")
+    private static void exit(final int status) {
+        System.exit(status);
+    }
+
+}

distribution/tools/java-version-checker/src/main/java/org/elasticsearch/tools/SuppressForbidden.java

@@ -0,0 +1,33 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch 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 org.elasticsearch.tools;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+/**
+ * Annotation to suppress forbidden-apis errors inside a whole class, a method, or a field.
+ */
+@Retention(RetentionPolicy.CLASS)
+@Target({ ElementType.CONSTRUCTOR, ElementType.FIELD, ElementType.METHOD, ElementType.TYPE })
+@interface SuppressForbidden {
+    String reason();
+}