Use StackWalker on Java 9+ and add compatibility layer

JavaCompat/pom.xml

@@ -0,0 +1,15 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+	<modelVersion>4.0.0</modelVersion>
+
+	<parent>
+		<groupId>com.gmail.filoghost.holographicdisplays</groupId>
+		<artifactId>holographicdisplays-parent</artifactId>
+		<version>2.3.0-SNAPSHOT</version>
+	</parent>
+
+	<artifactId>holographicdisplays-javacompat</artifactId>
+	<name>HolographicDisplays Java Compat</name>
+
+</project>

JavaCompat/src/main/java/java/lang/StackWalker.java

@@ -0,0 +1,20 @@
+package java.lang;
+
+import java.util.function.Function;
+import java.util.stream.Stream;
+
+public class StackWalker {
+	
+	public static interface StackFrame {
+		StackTraceElement toStackTraceElement();
+	}
+	
+	public static StackWalker getInstance() {
+		return null;
+	}
+	
+	public <T> T walk(Function<? super Stream<StackFrame>, ? extends T> function) {
+		return null;
+	}
+
+}

JavaCompat/src/main/java/java/util/Optional.java

@@ -0,0 +1,9 @@
+package java.util;
+
+public class Optional<T> {
+
+	public T orElse(T other) {
+		return null;
+	}
+
+}

JavaCompat/src/main/java/java/util/function/Function.java

@@ -0,0 +1,7 @@
+package java.util.function;
+
+public interface Function<T, R> {
+	
+	R apply(T t);
+
+}

JavaCompat/src/main/java/java/util/stream/Stream.java

@@ -0,0 +1,13 @@
+package java.util.stream;
+
+import java.util.Optional;
+
+public interface Stream<T> {
+
+	public Stream<T> skip(long n);
+
+	public Stream<T> limit(long maxSize);
+
+	public Optional<T> findFirst();
+
+}

Plugin/src/main/java/com/gmail/filoghost/holographicdisplays/HolographicDisplays.java

@@ -35,6 +35,7 @@ import com.gmail.filoghost.holographicdisplays.task.StartupLoadHologramsTask;
 import com.gmail.filoghost.holographicdisplays.task.WorldPlayerCounterTask;
 import com.gmail.filoghost.holographicdisplays.util.ConsoleLogger;
 import com.gmail.filoghost.holographicdisplays.util.NMSVersion;
+import com.gmail.filoghost.holographicdisplays.util.Utils;
 import com.gmail.filoghost.holographicdisplays.util.VersionUtils;
 
 public class HolographicDisplays extends JavaPlugin {
@@ -161,7 +162,7 @@ public class HolographicDisplays extends JavaPlugin {
 				if (requiredVersionError == null) {
 					ProtocolLibHook protocolLibHook;
 					
-					if (VersionUtils.classExists("com.comphenix.protocol.wrappers.WrappedDataWatcher$WrappedDataWatcherObject")) {
+					if (Utils.classExists("com.comphenix.protocol.wrappers.WrappedDataWatcher$WrappedDataWatcherObject")) {
 						// Only the new version contains this class
 						ConsoleLogger.log(Level.INFO, "Found ProtocolLib, using new version.");
 						protocolLibHook = new com.gmail.filoghost.holographicdisplays.bridge.protocollib.current.ProtocolLibHookImpl();

Utils/pom.xml

@@ -36,6 +36,13 @@
 			<version>1.8-R0.1-SNAPSHOT</version>
 			<optional>true</optional>
 		</dependency>
+		
+		<dependency>
+			<groupId>${project.groupId}</groupId>
+			<artifactId>holographicdisplays-javacompat</artifactId>
+			<version>2.3.0-SNAPSHOT</version>
+			<scope>provided</scope>
+		</dependency>
 	</dependencies>
 
 </project>

Utils/src/main/java/com/gmail/filoghost/holographicdisplays/util/Utils.java

@@ -53,15 +53,10 @@ public class Utils extends Object {
 		return new ArrayList<T>();
 	}
 	
-	
 	public static <T> Set<T> newSet() {
 		return new HashSet<T>();
 	}
 	
-	@SuppressWarnings("unchecked")
-	public static <T> T[] listToArray(List<T> list) {
-		return (T[]) list.toArray();
-	}
 	
 	public static int floor(double num) {
 		int floor = (int) num;
@@ -107,10 +102,12 @@ public class Utils extends Object {
 		return join(elements, separator, 0, elements.size());
 	}
 	
+	
 	public static String sanitize(String s) {
 		return s != null ? s : "null";
 	}
 	
+	
 	public static boolean isThereNonNull(Object... objects) {
 		if (objects == null) {
 			return false;
@@ -124,4 +121,14 @@ public class Utils extends Object {
 		
 		return false;
 	}
+	
+	
+	public static boolean classExists(String className) {
+		try {
+			Class.forName(className);
+			return true;
+		} catch (Throwable t) {
+			return false;
+		}
+	}
 }

Utils/src/main/java/com/gmail/filoghost/holographicdisplays/util/VersionUtils.java

@@ -79,14 +79,4 @@ public class VersionUtils {
 		return isVersionGreaterEqual(reference, lowest) && isVersionLessEqual(reference, highest);
 	}
 	
-
-	public static boolean classExists(String className) {
-		try {
-			Class.forName(className);
-			return true;
-		} catch (Throwable t) {
-			return false;
-		}
-	}
-	
 }

Utils/src/main/java/com/gmail/filoghost/holographicdisplays/util/reflection/ReflectionUtils.java

@@ -1,47 +1,69 @@
 package com.gmail.filoghost.holographicdisplays.util.reflection;
 
-import java.lang.reflect.Method;
+import java.lang.StackWalker.StackFrame;
+import java.util.function.Function;
 import java.util.logging.Level;
+import java.util.stream.Stream;
 
 import com.gmail.filoghost.holographicdisplays.util.ConsoleLogger;
+import com.gmail.filoghost.holographicdisplays.util.Utils;
 
 public class ReflectionUtils {
 	
-	private static Method getStackTraceElementMethod;
-	private static Method getStackTraceDepthMethod;
+	private static final boolean HAS_JAVA9_STACKWALKER = Utils.classExists("java.lang.StackWalker");
 	
-	private static boolean stackTraceErrorPrinted;
+	private static final ReflectMethod<Integer> GET_STACKTRACE_DEPTH_METHOD = new ReflectMethod<Integer>(Throwable.class, "getStackTraceDepth");
+	private static final ReflectMethod<StackTraceElement> GET_STACKTRACE_ELEMENT_METHOD = new ReflectMethod<StackTraceElement>(Throwable.class, "getStackTraceElement", int.class);
+	
+	private static boolean stackTraceError;
 	
 	
 	/**
 	 * If you only need one stack trace element this is faster than Throwable.getStackTrace()[element],
-	 * it doesn't generate the full stack trace.
+	 * it doesn't generate the full stack trace (except as fallback).
 	 */
-	public static StackTraceElement getStackTraceElement(int index) {
-		try {
-			if (getStackTraceElementMethod == null) {
-				getStackTraceElementMethod = Throwable.class.getDeclaredMethod("getStackTraceElement", int.class);
-				getStackTraceElementMethod.setAccessible(true);
-			}
-			if (getStackTraceDepthMethod == null) {
-				getStackTraceDepthMethod = Throwable.class.getDeclaredMethod("getStackTraceDepth");
-				getStackTraceDepthMethod.setAccessible(true);
-			}
+	public static StackTraceElement getStackTraceElement(final int index) {
+		// Use the faster methods only if there hasn't been any error while executing them previously
+		if (!stackTraceError) {
+			try {
+				if (HAS_JAVA9_STACKWALKER) {
+					// Use the Java 9 StackWalker API
+					StackFrame result = StackWalker.getInstance().walk(new Function<Stream<StackFrame>, StackFrame>() {
 	
-			Throwable dummyThrowable = new Throwable();
-			int depth = (Integer) getStackTraceDepthMethod.invoke(dummyThrowable);
+						@Override
+						public StackFrame apply(Stream<StackFrame> stream) {
+							return stream.skip(index).limit(1).findFirst().orElse(null);
+						}
 						
-			if (index < depth) {
-				return (StackTraceElement) getStackTraceElementMethod.invoke(new Throwable(), index);
-			} else {
-				return null;
-			}
-		} catch (Throwable t) {
-			if (!stackTraceErrorPrinted) {
-				ConsoleLogger.log(Level.WARNING, "Unable to get a stacktrace element, please inform the developer. You will only see this error once to avoid spam.", t);
-				stackTraceErrorPrinted = true;
+					});
+					return result != null ? result.toStackTraceElement() : null;
+					
+				} else {
+					// Use reflection to avoid generating the full stacktrace
+					Throwable dummyThrowable = new Throwable();
+					int depth = GET_STACKTRACE_DEPTH_METHOD.invoke(dummyThrowable);
+		
+					if (index < depth) {
+						return GET_STACKTRACE_ELEMENT_METHOD.invoke(dummyThrowable, index);
+					} else {
+						return null;
+					}
+				}
+			} catch (Throwable t) {
+				if (!stackTraceError) {
+					ConsoleLogger.log(Level.WARNING, "Unable to get a stacktrace element, please inform the developer. You will only see this error once and a fallback method will be used.", t);
+					stackTraceError = true;
+				}
 			}
+		}
+		
+		// Fallback slower method, generates the full stack trace (it should never be called if everything works as expected)
+		StackTraceElement[] fullStackTrace = new Throwable().getStackTrace();
+		if (index < fullStackTrace.length) {
+			return fullStackTrace[index];
+		} else {
 			return null;
 		}
+		
 	}
 }

pom.xml

@@ -18,6 +18,7 @@
 
 	<modules>
 		<module>API</module>
+		<module>JavaCompat</module>
 		<module>Utils</module>
 		<module>NMS/Interfaces</module>
 		<module>NMS/v1_8_R1</module>