OpenJDK 27 · java.lang.classfile.constantpool · APIdia, the JavaDoc alternative

Provides interfaces describing constant pool entries for the `java.lang.classfile` library.

The java.lang.classfile.constantpool package contains interfaces describing constant pool entries in the class file format. Constant pool entries are low-level models to faithfully represent the exact structure of a class file.

APIs in this package perform null and unrepresentable argument checks, unless otherwise noted.

Reading the constant pool entries

When read from class files, the pool entries are lazily inflated; the contents of these entries, besides the bare structure, are not evaluated to speed up parsing. Entries to users interest, usually accessed from other models and elements, have their contents read on demand. For example, to search for methods, a user should filter first by access flags and then by method name, and use Utf8Entry#equalsString(String) instead of checking equality against Utf8Entry#stringValue(). This avoids inflation of UTF-8 entries as much as possible:

boolean isStaticWorkMethod(MethodModel method) {
        // check static flag first to avoid unnecessary evaluation of UTF-8 entry
        return (method.flags().flagsMask() & ClassFile.ACC_STATIC) != 0
                // use equalsString to avoid full conversion to String for comparison
                // the Utf8Entry can also act as a basic CharSequence without full conversion
                && method.methodName().equalsString("work");
    }

The entries also define accessors to validated symbolic information with nominal descriptor abstractions from the java.lang.constant package. These symbolic information accessors perform validation against the read class files, and throw IllegalArgumentException when the accessed constant pool entry contains invalid data. The nominal descriptors represent validated data, which saves users from extra validations in future processing.

Due to the lazy nature of class file parsing, IllegalArgumentException indicating malformed class file data can be thrown at any method invocation. For example, an exception may come from a ClassEntry when it is first read from the constant pool (referring to an invalid index or wrong type of entry), when its referred UTF-8 entry is expanded (malformed UTF-8 data), or when its symbolic information is accessed (the string is not valid for a class entry).

Writing the constant pool entries

In general, users do not need to worry about working with the constant pool and its entries when writing class files. Most Class-File API models and elements have two sets of factory methods: one that accepts symbolic information representing the uses, and another that accepts constant pool entries. The constant pool builder associated with class file builders, ClassFileBuilder#constantPool, automatically creates or reuses pool entries from the symbolic information. Validated data in symbolic information helps class file generation by avoiding extraneous parsing of raw constant pool entry data.

As always, users can use factories that accept constant pool entries if they already have them by hand, or if they desire fine-grained control over class file generation.

If many models and elements are reused from another ClassModel in class building, the class building process can use a constant pool builder that extends from the given ClassModel, available through ConstantPoolBuilder::of(ClassModel), so that byte data with constant pool references can be copied in batch, speeding up class building. This is especially applicable to class transformations, and ConstantPoolSharingOption exists to control this behavior.

Interface Summary

Modifier and Type	Interface and Description
public interface	AnnotationConstantValueEntry Marker interface for constant pool entries that can represent constant values associated with elements of annotations.
public interface	ClassEntry Models a `CONSTANT_Class_info` structure, representing a reference type, in the constant pool of a `class` file.
public interface	ConstantDynamicEntry Models a `CONSTANT_Dynamic_info` structure, representing a dynamically-computed constant, in the constant pool of a `class` file.
public interface	ConstantPool Provides read access to the constant pool and the bootstrap method table of a `class` file.
public interface	ConstantPoolBuilder Builder for the constant pool of a `class` file.
public interface	ConstantValueEntry Marker interface for constant pool entries that can represent constant values in the `ConstantValue` attribute.
public interface	DoubleEntry Models a `CONSTANT_Double_info` structure, representing a `double` constant, in the constant pool of a `class` file.
public interface	DynamicConstantPoolEntry Superinterface modeling dynamically-computed constant pool entries, which include `ConstantDynamicEntry` and `InvokeDynamicEntry`, in the constant pool of a `class` file.
public interface	FieldRefEntry Models a `CONSTANT_Fieldref_info` structure, or a symbolic reference to a field, in the constant pool of a `class` file.
public interface	FloatEntry Models a `CONSTANT_Float_info` structure, or a `float` constant, in the constant pool of a `class` file.
public interface	IntegerEntry Models a `CONSTANT_Integer_info` structure, or an `int` constant, in the constant pool of a `class` file.
public interface	InterfaceMethodRefEntry Models a `CONSTANT_InterfaceMethodRef_info` structure, or a symbolic reference to an interface method, in the constant pool of a `class` file.
public interface	InvokeDynamicEntry Models a `CONSTANT_InvokeDynamic_info` structure, or the symbolic reference to a dynamically-computed call site, in the constant pool of a `class` file.
public interface	LoadableConstantEntry Marker interface for constant pool entries suitable for loading via the `ldc` instructions.
public interface	LongEntry Models a `CONSTANT_Long_info` structure, or a `long` constant, in the constant pool of a `class` file.
public interface	MemberRefEntry Superinterface modeling symbolic references to a member of a class or interface in the constant pool of a `class` file, which include references to fields, class methods, and interface methods.
public interface	MethodHandleEntry Models a `CONSTANT_MethodHandle_info` structure, or a symbolic reference to a method handle, in the constant pool of a `class` file.
public interface	MethodRefEntry Models a `CONSTANT_MethodRef_info` structure, or a symbolic reference to a class method, in the constant pool of a `class` file.
public interface	MethodTypeEntry Models a `CONSTANT_MethodType_info` structure, or a symbolic reference to a method type, in the constant pool of a `class` file.
public interface	ModuleEntry Models a `CONSTANT_Module_info` structure, denoting a module, in the constant pool of a `class` file.
public interface	NameAndTypeEntry Models a `CONSTANT_NameAndType_info` structure, representing a field or method, in the constant pool of a `class` file.
public interface	PackageEntry Models a `CONSTANT_Package_info`, representing a package, in the constant pool of a `class` file.
public interface	PoolEntry Models an entry in the constant pool of a `class` file.
public interface	StringEntry Models a `CONSTANT_String_info` structure, or a string constant, in the constant pool of a `class` file.
public interface	Utf8Entry Models a `CONSTANT_UTF8_info` constant, representing strings, in the constant pool of a `class` file.

Package java.lang.classfile.constantpool

Provides interfaces describing constant pool entries for the `java.lang.classfile` library.

Reading the constant pool entries

Writing the constant pool entries

Interface Summary

Class Summary

Package java.lang.classfile.constantpool

Provides interfaces describing constant pool entries for the java.lang.classfile library.

Reading the constant pool entries

Writing the constant pool entries

Interface Summary

Class Summary

Provides interfaces describing constant pool entries for the `java.lang.classfile` library.