Reflection Use in Native Images

Java reflection support (the java.lang.reflect.* API) enables Java code to examine its own classes, methods, fields and their properties at run time.

Native Image has partial support for reflection and needs to know ahead-of-time the reflectively accessed program elements. Examining and accessing program elements through java.lang.reflect.* or loading classes with Class.forName(String) at run time requires preparing additional metadata for those program elements. (Note: loading classes with Class.forName(String) are included here since it is closely related to reflection.)

Native Image tries to resolve the target elements through a static analysis that detects calls to the Reflection API. Where the analysis fails, the program elements reflectively accessed at run time must be specified using a manual configuration.

See also the guide on assisted configuration of Java resources and other dynamic features.

Automatic Detection #

The analysis intercepts calls to Class.forName(String), Class.forName(String, ClassLoader), Class.getDeclaredField(String), Class.getField(String), Class.getDeclaredMethod(String, Class[]), Class.getMethod(String, Class[]), Class.getDeclaredConstructor(Class[]), and Class.getConstructor(Class[]).

If the arguments to these calls can be reduced to a constant, Native Image tries to resolve the target elements. If the target elements can be resolved, the calls are removed and instead the target elements are embedded in the code. If the target elements cannot be resolved, e.g., a class is not on the classpath or it does not declare a field/method/constructor, then the calls are replaced with a snippet that throws the appropriate exception at run time. The benefits are twofold. First, at run time there are no calls to the Reflection API. Second, GraalVM can employ constant folding and optimize the code further.

The calls are intercepted and processed only when it can be unequivocally determined that the parameters can be reduced to a constant. For example, the call Class.forName(String) will be replaced with a Class literal only if the String argument can be constant folded, assuming that the class is actually on the classpath. Additionally, a call to Class.getMethod(String, Class[]) will be processed only if the contents of the Class[] argument can be determined with certainty. This last restriction is due to the fact that Java does not have immutable arrays. Therefore, all the changes to the array between the time it is allocated and the time it is passed as an argument need to be tracked. The analysis follows a simple rule: if all the writes to the array happen in linear sections of code, i.e., no control flow splits, then the array is effectively constant for the purpose of analyzing the call.

That is why the analysis does not accept Class[] arguments coming from static fields, since the contents of those can change at any time, even if the fields are final. Although this may seem too restrictive, it covers the most commonly used patterns of the Reflection API calls.

The only exception to the constant arguments rule is that the ClassLoader argument of Class.forName(String, ClassLoader) does not need to be a constant; it is ignored and instead a class loader that can load all the classes on the class path is used. The analysis runs to a fix point which means that a chain of calls like Class.forName(String).getMethod(String, Class[]) will first replace the class constant and then the method will effectively reduce this to java.lang.reflect.Method.

Following are examples of calls that can be intercepted and replaced with the corresponding element:

Class.forName("java.lang.Integer", true, ClassLoader.getSystemClassLoader())
Class.forName("java.lang.Integer").getMethod("equals", Object.class)
Integer.class.getDeclaredMethod("bitCount", int.class)

The following ways to declare and populate an array are equivalent from the point of view of the analysis:

Class<?>[] params0 = new Class<?>[]{String.class, int.class};
Integer.class.getMethod("parseInt", params0);
Class<?>[] params1 = new Class<?>[2];
params1[0] = Class.forName("java.lang.String");
params1[1] = int.class;
Integer.class.getMethod("parseInt", params1);
Class<?>[] params2 = {String.class, int.class};
Integer.class.getMethod("parseInt", params2);

If a call cannot be processed it is simply skipped. For these situations a manual configuration as described below can be provided.

Manual Configuration #

A configuration that specifies the program elements that will be accessed reflectively can be provided during the native image build as follows:


Here, reflectconfig is a JSON file in the following format (use --expert-options for more details):

    "name" : "java.lang.Class",
    "allDeclaredConstructors" : true,
    "allPublicConstructors" : true,
    "allDeclaredMethods" : true,
    "allPublicMethods" : true,
    "allDeclaredClasses" : true,
    "allPublicClasses" : true
    "name" : "java.lang.String",
    "fields" : [
      { "name" : "value", "allowWrite" : true },
      { "name" : "hash" }
    "methods" : [
      { "name" : "<init>", "parameterTypes" : [] },
      { "name" : "<init>", "parameterTypes" : ["char[]"] },
      { "name" : "charAt" },
      { "name" : "format", "parameterTypes" : ["java.lang.String", "java.lang.Object[]"] }
      "name" : "java.lang.String$CaseInsensitiveComparator",
      "methods" : [
        { "name" : "compare" }

The native image builder generates reflection metadata for all classes, methods, and fields referenced in that file. The allPublicConstructors, allDeclaredConstructors, allPublicMethods, allDeclaredMethods, allPublicFields, allDeclaredFields, allPublicClasses, and allDeclaredClasses attributes can be used to automatically include an entire set of members of a class.

However, allPublicClasses and allDeclaredClasses do not automatically register the inner classes for reflective access. They just make them available via Class.getClasses() and Class.getDeclaredClasses() when called on the declaring class. In order to write a field that is declared final, the allowWrite attribute must be specified for that field (but is not required for non-final fields). However, code accessing final fields might not observe changes of final field values in the same way as for non-final fields because of optimizations.

More than one configuration can be used by specifying multiple paths for ReflectionConfigurationFiles and separating them with ,. Also, -H:ReflectionConfigurationResources can be specified to load one or several configuration files from the native image build’s class path, such as from a JAR file.

Alternatively, a custom Feature implementation can register program elements before and during the analysis phase of the native image build using the RuntimeReflection class. For example:

class RuntimeReflectionRegistrationFeature implements Feature {
  public void beforeAnalysis(BeforeAnalysisAccess access) {
    try {
      RuntimeReflection.register(/* finalIsWritable: */ true, String.class.getDeclaredField("value"));
      RuntimeReflection.register(String.class.getDeclaredMethod("charAt", int.class));
      RuntimeReflection.register(String.class.getDeclaredMethod("format", String.class, Object[].class));
      RuntimeReflection.register(String.CaseInsensitiveComparator.class.getDeclaredMethod("compare", String.class, String.class));
    } catch (NoSuchMethodException | NoSuchFieldException e) { ... }

Use of Reflection during Native Image Generation #

Reflection can be used without restrictions during a native image generation, for example, in static initializers. At this point, code can collect information about methods and fields and store them in their own data structures, which are then reflection-free at run time.

Unsafe Accesses #

The Unsafe class, although its use is discouraged, provides direct access to memory of Java objects. The Unsafe.objectFieldOffset() method provides the offset of a field within a Java object. This information is not available by default, but can be enabled with the allowUnsafeAccess attribute in the reflection configuration. For example:

"fields" : [ { "name" : "hash", "allowUnsafeAccess" : true }, ... ]

Note: The offsets that are queried during a native image generation can be different from the offsets at run time.