Recover from error during SpEL MIXED mode compilation

Prior to this commit, SpEL was able to recover from an error that
occurred while running a CompiledExpression; however, SpEL was not able
to recover from an error that occurred while compiling the expression
(such as a java.lang.VerifyError). The latter can occur when multiple
threads concurrently change types involved in the expression, such as
the concrete type of a custom variable registered via
EvaluationContext.setVariable(...), which can result in SpEL generating
invalid bytecode.

This commit addresses this issue by catching exceptions thrown while
compiling an expression and updating the `failedAttempts` and
`interpretedCount` counters accordingly. If an exception is caught
while operating in SpelCompilerMode.IMMEDIATE mode, the exception will
be propagated via a SpelEvaluationException with a new
SpelMessage.EXCEPTION_COMPILING_EXPRESSION error category.

Closes gh-28043

spring-expression/src/main/java/org/springframework/expression/spel/SpelMessage.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 the original author or authors.
+ * Copyright 2002-2022 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.
@@ -27,10 +27,11 @@ import java.text.MessageFormat;
  * <p>When a message is formatted, it will have this kind of form, capturing the prefix
  * and the error kind:
  *
- * <pre class="code">EL1004E: Type cannot be found 'String'</pre>
+ * <pre class="code">EL1005E: Type cannot be found 'String'</pre>
  *
  * @author Andy Clement
  * @author Juergen Hoeller
+ * @author Sam Brannen
  * @since 3.0
  */
 public enum SpelMessage {
@@ -255,7 +256,11 @@ public enum SpelMessage {
 
 	/** @since 4.3.17 */
 	FLAWED_PATTERN(Kind.ERROR, 1073,
-			"Failed to efficiently evaluate pattern ''{0}'': consider redesigning it");
+			"Failed to efficiently evaluate pattern ''{0}'': consider redesigning it"),
+
+	/** @since 5.3.16 */
+	EXCEPTION_COMPILING_EXPRESSION(Kind.ERROR, 1074,
+			"An exception occurred while compiling an expression");
 
 
 	private final Kind kind;

spring-expression/src/main/java/org/springframework/expression/spel/standard/SpelCompiler.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2022 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.
@@ -110,7 +110,8 @@ public final class SpelCompiler implements Opcodes {
 					return ReflectionUtils.accessibleConstructor(clazz).newInstance();
 				}
 				catch (Throwable ex) {
-					throw new IllegalStateException("Failed to instantiate CompiledExpression", ex);
+					throw new IllegalStateException("Failed to instantiate CompiledExpression for expression: " +
+							expression.toStringAST(), ex);
 				}
 			}
 		}

spring-expression/src/main/java/org/springframework/expression/spel/standard/SpelExpression.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2022 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.
@@ -44,6 +44,7 @@ import org.springframework.util.Assert;
  *
  * @author Andy Clement
  * @author Juergen Hoeller
+ * @author Sam Brannen
  * @since 3.0
  */
 public class SpelExpression implements Expression {
@@ -522,17 +523,34 @@ public class SpelExpression implements Expression {
 				// Compiled by another thread before this thread got into the sync block
 				return true;
 			}
-			SpelCompiler compiler = SpelCompiler.getCompiler(this.configuration.getCompilerClassLoader());
-			compiledAst = compiler.compile(this.ast);
-			if (compiledAst != null) {
-				// Successfully compiled
-				this.compiledAst = compiledAst;
-				return true;
+			try {
+				SpelCompiler compiler = SpelCompiler.getCompiler(this.configuration.getCompilerClassLoader());
+				compiledAst = compiler.compile(this.ast);
+				if (compiledAst != null) {
+					// Successfully compiled
+					this.compiledAst = compiledAst;
+					return true;
+				}
+				else {
+					// Failed to compile
+					this.failedAttempts.incrementAndGet();
+					return false;
+				}
 			}
-			else {
+			catch (Exception ex) {
 				// Failed to compile
 				this.failedAttempts.incrementAndGet();
-				return false;
+
+				// If running in mixed mode, revert to interpreted
+				if (this.configuration.getCompilerMode() == SpelCompilerMode.MIXED) {
+					this.compiledAst = null;
+					this.interpretedCount.set(0);
+					return false;
+				}
+				else {
+					// Running in SpelCompilerMode.immediate mode - propagate exception to caller
+					throw new SpelEvaluationException(ex, SpelMessage.EXCEPTION_COMPILING_EXPRESSION);
+				}
 			}
 		}
 	}

spring-expression/src/test/java/org/springframework/expression/spel/standard/SpelCompilerTests.java

@@ -76,6 +76,21 @@ class SpelCompilerTests {
 		assertThat(expression.getValue(context)).asInstanceOf(BOOLEAN).isTrue();
 	}
 
+	@Test  // gh-28043
+	void changingRegisteredVariableTypeDoesNotResultInFailureInMixedMode() {
+		SpelParserConfiguration config = new SpelParserConfiguration(SpelCompilerMode.MIXED, null);
+		SpelExpressionParser parser = new SpelExpressionParser(config);
+		Expression sharedExpression = parser.parseExpression("#bean.value");
+		StandardEvaluationContext context = new StandardEvaluationContext();
+
+		Object[] beans = new Object[] {new Bean1(), new Bean2(), new Bean3(), new Bean4()};
+
+		IntStream.rangeClosed(1, 1_000_000).parallel().forEach(count -> {
+			context.setVariable("bean", beans[count % 4]);
+			assertThat(sharedExpression.getValue(context)).asString().startsWith("1");
+		});
+	}
+
 
 	static class OrderedComponent implements Ordered {
 
@@ -121,4 +136,28 @@ class SpelCompilerTests {
 		boolean hasSomeProperty();
 	}
 
+	public static class Bean1 {
+		public String getValue() {
+			return "11";
+		}
+	}
+
+	public static class Bean2 {
+		public Integer getValue() {
+			return 111;
+		}
+	}
+
+	public static class Bean3 {
+		public Float getValue() {
+			return 1.23f;
+		}
+	}
+
+	public static class Bean4 {
+		public Character getValue() {
+			return '1';
+		}
+	}
+
 }