java.lang.classfile library.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. Unless otherwise specified, passing null or an array or collection containing a null element as an argument to a constructor or method of any Class-File API class or interface will cause a NullPointerException to be thrown.
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).
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.
| Class | Description |
|---|---|
| AnnotationConstantValueEntry | Marker interface for constant pool entries that can represent constant values associated with elements of annotations. |
| ClassEntry | Models a CONSTANT_Class_info structure, representing a reference type, in the constant pool of a class file. |
| ConstantDynamicEntry | Models a CONSTANT_Dynamic_info structure, representing a dynamically-computed constant, in the constant pool of a
class file. |
| ConstantPool | Provides read access to the constant pool and the bootstrap method table of a class file. |
| ConstantPoolBuilder | Builder for the constant pool of a class file. |
| ConstantPoolException | Thrown to indicate that requested entry cannot be obtained from the constant pool or the bootstrap method table. |
| ConstantValueEntry | Marker interface for constant pool entries that can represent constant values in the ConstantValue attribute. |
| DoubleEntry | Models a CONSTANT_Double_info structure, representing a
double constant, in the constant pool of a class file. |
| DynamicConstantPoolEntry | Superinterface modeling dynamically-computed constant pool entries, which include ConstantDynamicEntry and InvokeDynamicEntry, in the constant pool of a class file. |
| FieldRefEntry | Models a CONSTANT_Fieldref_info structure, or a symbolic reference to a field, in the constant pool of a class file. |
| FloatEntry | Models a CONSTANT_Float_info structure, or a float constant, in the constant pool of a class file. |
| IntegerEntry | Models a CONSTANT_Integer_info structure, or an int constant, in the constant pool of a class file. |
| InterfaceMethodRefEntry | Models a CONSTANT_InterfaceMethodRef_info structure, or a symbolic reference to an interface method, in the constant pool of a class file. |
| 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. |
| LoadableConstantEntry | Marker interface for constant pool entries suitable for loading via the ldc instructions. |
| LongEntry | Models a CONSTANT_Long_info structure, or a long constant, in the constant pool of a class file. |
| 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. |
| MethodHandleEntry | Models a CONSTANT_MethodHandle_info structure, or a symbolic reference to a method handle, in the constant pool of a class file. |
| MethodRefEntry | Models a CONSTANT_MethodRef_info structure, or a symbolic reference to a class method, in the constant pool of a class file. |
| MethodTypeEntry | Models a CONSTANT_MethodType_info structure, or a symbolic reference to a method type, in the constant pool of a class file. |
| ModuleEntry | Models a CONSTANT_Module_info structure, denoting a module, in the constant pool of a class file. |
| NameAndTypeEntry | Models a CONSTANT_NameAndType_info structure, representing a field or method, in the constant pool of a class file. |
| PackageEntry | Models a CONSTANT_Package_info, representing a package, in the constant pool of a class file. |
| PoolEntry | Models an entry in the constant pool of a class file. |
| StringEntry | Models a CONSTANT_String_info structure, or a string constant, in the constant pool of a class file. |
| Utf8Entry | Models a CONSTANT_UTF8_info constant, representing strings, in the constant pool of a class file. |
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