LocalGregorianCalendarReport.java

LocalGregorianCalendarReport.java

import java.io.IOException;
import java.io.InputStream;
import java.lang.reflect.Proxy;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.List;
import java.util.Objects;
import java.util.OptionalInt;
import java.util.function.Predicate;
import java.util.function.ToIntFunction;
import java.util.stream.IntStream;
import java.util.stream.Stream;

import org.apache.bcel.classfile.ClassParser;
import org.apache.bcel.classfile.Method;
import org.apache.bcel.generic.ConstantPoolGen;
import org.apache.bcel.generic.ICONST;
import org.apache.bcel.generic.Instruction;
import org.apache.bcel.generic.LDC;
import org.apache.bcel.generic.LDC2_W;
import org.apache.bcel.generic.MethodGen;
import org.apache.bcel.generic.PUTSTATIC;
import org.apache.commons.collections4.IterableUtils;
import org.apache.commons.lang3.ArrayUtils;
import org.apache.commons.lang3.ObjectUtils;
import org.apache.commons.lang3.StringUtils;

public class LocalGregorianCalendarReport {

	private static class Era {

		private String name, abbr;

		private Long since;

		private Boolean localTime;

	}

	public static void main(final String[] args)
			throws ClassNotFoundException, NoSuchFieldException, IllegalAccessException, IOException {
		//
		List<Era> list = null;
		//
		Era era = null;
		//
		try (final InputStream is = getResourceAsStream(Object.class,
				"/sun/util/calendar/LocalGregorianCalendar.class")) {
			//
			final List<Method> ms = toList(filter(Arrays.stream(new ClassParser(is, null).parse().getMethods()),
					m -> m != null ? Objects.equals(m.getName(), "<clinit>") : null));
			//
			final int size = IterableUtils.size(ms);
			//
			if (size > 1) {
				//
				throw new IllegalStateException();
				//
			} // if
				//
			final Method m = size == 1 ? IterableUtils.get(ms, 0) : null;
			//
			final Instruction[] ins = m != null ? new MethodGen(m, null, null).getInstructionList().getInstructions()
					: null;
			//
			final int length = ins != null ? ins.length : 0;
			//
			Integer index = null;
			//
			for (int i = 0; ins != null && i < length; i++) {
				//
				if (!(ArrayUtils.get(ins, i) instanceof PUTSTATIC)) {
					//
					continue;
					//
				} // if
					//
				if (index != null) {
					//
					throw new IllegalStateException();
					//
				} // if
					//
				index = Integer.valueOf(i);
				//
			} // for
				//
			Instruction in = null;
			//
			ConstantPoolGen cpg = null;
			//
			Number number = null;
			//
			for (int i = 0; ins != null && i < Math.max(length, intValue(index, 0)); i++) {
				//
				if (cpg == null) {
					//
					cpg = new ConstantPoolGen(m.getConstantPool());
					//
				} // if
					//
				if ((in = ArrayUtils.get(ins, i)) instanceof LDC ldc && ldc != null) {
					//
					if (i > 0 && ArrayUtils.get(ins, i - 1) instanceof LDC) {
						//
						if (era != null) {
							//
							era.abbr = Objects.toString(ldc.getValue(cpg));
							//
						} // if
							//
					} else {
						//
						(era = new Era()).name = Objects.toString(ldc.getValue(cpg));
						//
						add(list = ObjectUtils.getIfNull(list, ArrayList::new), era);
						//
					} // if
						//
				} else if (in instanceof LDC2_W ldc2w && ldc2w != null && era != null) {
					//
					era.since = (number = ldc2w.getValue(cpg)) != null ? Long.valueOf(number.longValue()) : null;
					//
				} else if (in instanceof ICONST iconst && iconst != null && i > 0
						&& ArrayUtils.get(ins, i - 1) instanceof LDC2_W && era != null) {
					//
					if (intValue(number = iconst.getValue(), -1) == 1) {
						//
						era.localTime = true;
						//
					} else {
						//
						throw new IllegalStateException();
						//
					} // if
						//
				} // if
					//
			} // for
				//
		} // try
			//
		final int maxNameLength = orElse(
				max(mapToInt(stream(list), x -> StringUtils.length(x != null ? x.name : null))), 0);
		//
		final int maxSinceLength = orElse(
				max(mapToInt(stream(list), x -> StringUtils.length(Objects.toString(x != null ? x.since : null)))), 0);
		//
		for (int i = 0; i < IterableUtils.size(list); i++) {
			//
			if ((era = IterableUtils.get(list, i)) == null) {
				//
				continue;
				//
			} // if
				//
			System.out.println(StringUtils.joinWith(" ", StringUtils.rightPad(era.name, maxNameLength), era.abbr,
					StringUtils.leftPad(Objects.toString(era.since), maxSinceLength), era.localTime));
			//
		} // for
			//
	}

	private static int orElse(final OptionalInt instance, final int other) {
		return instance != null ? instance.orElse(other) : other;
	}

	private static OptionalInt max(final IntStream instance) {
		return instance != null ? instance.max() : null;
	}

	private static <T> IntStream mapToInt(final Stream<T> instance, final ToIntFunction<? super T> mapper) {
		//
		return instance != null && (Proxy.isProxyClass(getClass(instance)) || mapper != null)
				? instance.mapToInt(mapper)
				: null;
		//
	}

	private static Class<?> getClass(final Object instance) {
		return instance != null ? instance.getClass() : null;
	}

	private static <T> Stream<T> stream(final Collection<T> instance) {
		return instance != null ? instance.stream() : null;
	}

	private static <E> void add(final Collection<E> instance, final E item) {
		if (instance != null) {
			instance.add(item);
		}
	}

	private static int intValue(final Number instance, final int defaultValue) {
		return instance != null ? instance.intValue() : defaultValue;
	}

	private static <T> List<T> toList(final Stream<T> instance) {
		return instance != null ? instance.toList() : null;
	}

	private static <T> Stream<T> filter(final Stream<T> instance, final Predicate<? super T> predicate) {
		return instance != null ? instance.filter(predicate) : instance;
	}

	private static InputStream getResourceAsStream(final Class<?> instance, final String name) {
		return instance != null ? instance.getResourceAsStream(name) : null;
	}

}

pom.xml

<!-- https://mvnrepository.com/artifact/org.apache.bcel/bcel -->
<dependency>
	<groupId>org.apache.bcel</groupId>
	<artifactId>bcel</artifactId>
	<version>6.12.0</version>
</dependency>
<!-- https://mvnrepository.com/artifact/org.apache.commons/commons-lang3 -->
<dependency>
	<groupId>org.apache.commons</groupId>
	<artifactId>commons-lang3</artifactId>
	<version>3.20.0</version>
</dependency>
<!--https://mvnrepository.com/artifact/org.apache.commons/commons-collections4 -->
<dependency>
	<groupId>org.apache.commons</groupId>
	<artifactId>commons-collections4</artifactId>
	<version>4.5.0</version>
</dependency>

https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/sun/util/calendar/LocalGregorianCalendar.java#L39

Sample Output

Meiji  M -3218832000000 true
Taisho T -1812153600000 true
Showa  S -1357603200000 true
Heisei H   600220800000 true
Reiwa  R  1556668800000 true