Improve type resolution cache for classes in java package (#7714)

Classes in `java.` package can only be loaded by boot loader. As the
class loader that we currently use in cache key for these classes isn't
always the boot loader we can end up with multiple entries for the same
class.

muzzle/src/main/java/io/opentelemetry/javaagent/tooling/muzzle/AgentCachingPoolStrategy.java

@@ -153,7 +153,7 @@ private TypePool.CacheProvider getCacheProvider(ClassLoader classLoader) {
     WeakReference<ClassLoader> loaderRef =
         loaderRefCache.computeIfAbsent(classLoader, WeakReference::new);
 
-    int loaderHash = classLoader.hashCode();
+    int loaderHash = System.identityHashCode(classLoader);
     return new SharedResolutionCacheAdapter(loaderHash, loaderRef, sharedResolutionCache);
   }
 
@@ -168,17 +168,19 @@ private TypePool.CacheProvider getCacheProvider(ClassLoader classLoader) {
    */
   private static final class TypeCacheKey {
     private final int loaderHash;
-    private final WeakReference<ClassLoader> loaderRef;
+    @Nullable private final WeakReference<ClassLoader> loaderRef;
     private final String className;
 
     private final int hashCode;
 
     TypeCacheKey(int loaderHash, WeakReference<ClassLoader> loaderRef, String className) {
-      this.loaderHash = loaderHash;
-      this.loaderRef = loaderRef;
+      // classes in java package are always loaded from boot loader
+      // set loader to boot loader to avoid creating multiple cache entries
+      this.loaderHash = className.startsWith("java.") ? BOOTSTRAP_HASH : loaderHash;
+      this.loaderRef = className.startsWith("java.") ? null : loaderRef;
       this.className = className;
 
-      hashCode = 31 * loaderHash + className.hashCode();
+      hashCode = 31 * this.loaderHash + className.hashCode();
     }
 
     @Override
@@ -204,6 +206,8 @@ public boolean equals(@Nullable Object obj) {
       // Also covers the bootstrap null loaderRef case
       if (loaderRef == other.loaderRef) {
         return true;
+      } else if (loaderRef == null || other.loaderRef == null) {
+        return false;
       }
 
       // need to perform a deeper loader check -- requires calling Reference.get
@@ -268,16 +272,16 @@ private static final class SharedResolutionCacheAdapter implements TypePool.Cach
 
     @Override
     public TypePool.Resolution find(String className) {
+      if (OBJECT_NAME.equals(className)) {
+        return OBJECT_RESOLUTION;
+      }
+
       TypePool.Resolution existingResolution =
           sharedResolutionCache.get(new TypeCacheKey(loaderHash, loaderRef, className));
       if (existingResolution != null) {
         return existingResolution;
       }
 
-      if (OBJECT_NAME.equals(className)) {
-        return OBJECT_RESOLUTION;
-      }
-
       return null;
     }
 
@@ -666,7 +670,7 @@ public String getName() {
     @Override
     public boolean isAnnotation() {
       // by default byte-buddy checks whether class modifiers have annotation bit set
-      // as we wish to avoid looking up the class bytes we assume that every that was expected
+      // as we wish to avoid looking up the class bytes we assume that every class that was expected
       // to be an annotation really is an annotation and return true here
       // See TypePool.Default.LazyTypeDescription.LazyAnnotationDescription.asList()
       return true;