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/OpenJDK 25

Interface ClassReader

All Superinterfaces:
ConstantPool, Iterable<PoolEntry>
public sealed interface ClassReader extends ConstantPool
Advanced class file reading support for AttributeMappers. Supports reading arbitrary offsets within a class file and reading data of various numeric types (e.g., u2, u4) in addition to constant pool access.

All numeric values in the class file format are big endian.

Unless otherwise specified, all out-of-bounds access result in an IllegalArgumentException to indicate the class file data is malformed. Since the class file data is arbitrary, users should sanity-check the structural integrity of the data before attempting to interpret the potentially malformed data.

Since:
24
See Also:

Method Summary

Modifier and Type Method Description
int classfileLength()
Returns the length of the class file, in number of bytes.
void copyBytesTo(BufWriter buf, int offset, int len)
Copy a range of bytes from the class file to a BufWriter.
Function<Utf8Entry, AttributeMapper<?>> customAttributes()
Returns the table of custom attribute mappers.
int flags()
Returns the access flags for the class, as a bit mask.
byte[] readBytes(int offset, int len)
Returns a copy of the bytes at the specified range in the class file.
double readDouble(int offset)
Returns the double value at the specified offset within the class file.
PoolEntry readEntry(int offset)
Returns the constant pool entry whose index is given at the specified offset within the class file.
<T extends PoolEntry>
T		 readEntry(int offset, Class<T> cls)
Returns the constant pool entry of a given type whose index is given at the specified offset within the class file.
PoolEntry readEntryOrNull(int offset)
Returns the constant pool entry whose index is given at the specified offset within the class file, or null if the index at the specified offset is zero.
<T extends PoolEntry>
T		 readEntryOrNull(int offset, Class<T> cls)
Returns the constant pool entry of a given type whose index is given at the specified offset within the class file, or null if the index at the specified offset is zero.
float readFloat(int offset)
Returns the float value at the specified offset within the class file.
int readInt(int offset)
Returns the signed int at the specified offset within the class file.
long readLong(int offset)
Returns the signed long at the specified offset within the class file.
int readS1(int offset)
Returns the signed byte at the specified offset within the class file.
int readS2(int offset)
Returns the signed byte at the specified offset within the class file.
int readU1(int offset)
Returns the unsigned byte at the specified offset within the class file.
int readU2(int offset)
Returns the unsigned short at the specified offset within the class file.
Optional<ClassEntry> superclassEntry()
Returns the constant pool entry describing the name of the superclass, if any.
ClassEntry thisClassEntry()
Returns the constant pool entry describing the name of class.

Methods declared in interface ConstantPool

bootstrapMethodCount, bootstrapMethodEntry, entryByIndex, entryByIndex, iterator, size

Methods declared in interface Iterable

forEach, spliterator

Method Details

customAttributes

Function<Utf8Entry, AttributeMapper<?>> customAttributes()
Returns the table of custom attribute mappers. This is derived from the processing option ClassFile.AttributeMapperOption.
Returns:
the table of custom attribute mappers

flags

int flags()
Returns the access flags for the class, as a bit mask.
Returns:
the access flags for the class, as a bit mask
See Also:

thisClassEntry

ClassEntry thisClassEntry()
Returns the constant pool entry describing the name of class.
Returns:
the constant pool entry describing the name of class
See Also:

superclassEntry

Optional<ClassEntry> superclassEntry()
Returns the constant pool entry describing the name of the superclass, if any.
Returns:
the constant pool entry describing the name of the superclass, if any
See Also:

classfileLength

int classfileLength()
Returns the length of the class file, in number of bytes.
Returns:
the length of the class file, in number of bytes

readEntry

PoolEntry readEntry(int offset)
Returns the constant pool entry whose index is given at the specified offset within the class file.
API Note:
If only a particular type of entry is expected, use readEntry(int, Class).
Parameters:
offset - the offset of the index within the class file
Returns:
the constant pool entry whose index is given at the specified offset within the class file
Throws:
ConstantPoolException - if the index is out of range of the constant pool size, or zero

readEntry

<T extends PoolEntry> T readEntry(int offset, Class<T> cls)
Returns the constant pool entry of a given type whose index is given at the specified offset within the class file.
Type Parameters:
T - the entry type
Parameters:
offset - the offset of the index within the class file
cls - the entry type
Returns:
the constant pool entry of a given type whose index is given at the specified offset within the class file
Throws:
ConstantPoolException - if the index is out of range of the constant pool size, or zero, or the entry is not of the given type

readEntryOrNull

PoolEntry readEntryOrNull(int offset)
Returns the constant pool entry whose index is given at the specified offset within the class file, or null if the index at the specified offset is zero.
API Note:
If only a particular type of entry is expected, use readEntryOrNull(int, Class).
Parameters:
offset - the offset of the index within the class file
Returns:
the constant pool entry whose index is given at the specified offset within the class file, or null if the index at the specified offset is zero
Throws:
ConstantPoolException - if the index is out of range of the constant pool size

readEntryOrNull

<T extends PoolEntry> T readEntryOrNull(int offset, Class<T> cls)
Returns the constant pool entry of a given type whose index is given at the specified offset within the class file, or null if the index at the specified offset is zero.
Type Parameters:
T - the entry type
Parameters:
offset - the offset of the index within the class file
cls - the entry type
Returns:
the constant pool entry of a given type whose index is given at the specified offset within the class file, or null if the index at the specified offset is zero
Throws:
ConstantPoolException - if the index is out of range of the constant pool size, or zero, or the entry is not of the given type

readU1

int readU1(int offset)
Returns the unsigned byte at the specified offset within the class file. Reads a byte and zero-extends it to an int.
Parameters:
offset - the offset within the class file
Returns:
the unsigned byte at the specified offset within the class file

readU2

int readU2(int offset)
Returns the unsigned short at the specified offset within the class file. Reads a 2-byte value and zero-extends it to an int.
Parameters:
offset - the offset within the class file
Returns:
the unsigned short at the specified offset within the class file

readS1

int readS1(int offset)
Returns the signed byte at the specified offset within the class file. Reads a byte and sign-extends it to an int.
Parameters:
offset - the offset within the class file
Returns:
the signed byte at the specified offset within the class file

readS2

int readS2(int offset)
Returns the signed byte at the specified offset within the class file. Reads a 2-byte value and sign-extends it to an int.
Parameters:
offset - the offset within the class file
Returns:
the signed byte at the specified offset within the class file

readInt

int readInt(int offset)
Returns the signed int at the specified offset within the class file. Reads 4 bytes of value.
Parameters:
offset - the offset within the class file
Returns:
the signed int at the specified offset within the class file

readLong

long readLong(int offset)
Returns the signed long at the specified offset within the class file. Reads 8 bytes of value.
Parameters:
offset - the offset within the class file
Returns:
the signed long at the specified offset within the class file

readFloat

float readFloat(int offset)
Returns the float value at the specified offset within the class file. Reads 4 bytes of value.

In the conversions, all NaN values of the float may or may not be collapsed into a single "canonical" NaN value.

Parameters:
offset - the offset within the class file
Returns:
the float value at the specified offset within the class file

readDouble

double readDouble(int offset)
Returns the double value at the specified offset within the class file. Reads 8 bytes of value.

In the conversions, all NaN values of the double may or may not be collapsed into a single "canonical" NaN value.

Parameters:
offset - the offset within the class file
Returns:
the double value at the specified offset within the class file

readBytes

byte[] readBytes(int offset, int len)
Returns a copy of the bytes at the specified range in the class file.
Parameters:
offset - the offset within the class file
len - the length of the range
Returns:
a copy of the bytes at the specified range in the class file

copyBytesTo

void copyBytesTo(BufWriter buf, int offset, int len)
Copy a range of bytes from the class file to a BufWriter.
Parameters:
buf - the BufWriter
offset - the offset within the class file
len - the length of the range

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Documentation extracted from Debian's OpenJDK Development Kit package.
Licensed under the GNU General Public License, version 2, with the Classpath Exception.
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https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/lang/classfile/ClassReader.html