JapaneseEraSinceMap.java

import java.io.IOException;
import java.io.InputStream;
import java.time.LocalDate;
import java.time.chrono.JapaneseEra;
import java.util.Arrays;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.Objects;

import org.apache.bcel.classfile.ClassParser;
import org.apache.bcel.classfile.JavaClass;
import org.apache.bcel.classfile.Method;
import org.apache.bcel.generic.BIPUSH;
import org.apache.bcel.generic.ConstantPoolGen;
import org.apache.bcel.generic.ICONST;
import org.apache.bcel.generic.Instruction;
import org.apache.bcel.generic.MethodGen;
import org.apache.bcel.generic.PUTSTATIC;
import org.apache.bcel.generic.SIPUSH;
import org.apache.commons.lang3.ObjectUtils;
import org.apache.commons.lang3.StringUtils;

public class JapaneseEraSinceMap {

	public static void main(final String[] args) {
		//
		// https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/time/chrono/JapaneseEra.java
		//
		// {Meiji=1868-01-01, Taisho=1912-07-30, Showa=1926-12-25, Heisei=1989-01-08,
		// Reiwa=2019-05-01}
		//
		System.out.println(getJapaneseEraSinceMap());
		//
	}

	private static Map<JapaneseEra, LocalDate> getJapaneseEraSinceMap() {
		//
		Map<JapaneseEra, LocalDate> map = null;
		//
		final Class<?> clz = JapaneseEra.class;
		//
		try (final InputStream is = clz != null
				? clz.getResourceAsStream(String.format("/%1$s.class", StringUtils.replace(clz.getName(), ".", "/")))
				: null) {
			//
			final JavaClass jc = new ClassParser(is, null).parse();
			//
			final List<Method> ms = Arrays.stream(jc.getMethods())
					.filter(m -> m != null ? Objects.equals(m.getName(), "<clinit>") : null).toList();
			//
			if (ms != null && ms.size() > 1) {
				//
				throw new IllegalStateException();
				//
			} // if
				//
			final Method m = ms != null && ms.size() == 1 ? ms.get(0) : null;
			//
			final Instruction[] ins = m != null ? new MethodGen(m, null, null).getInstructionList().getInstructions()
					: null;
			//
			Instruction in = null;
			//
			SIPUSH sipush = null;
			//
			PUTSTATIC putstatic = null;
			// s
			ConstantPoolGen cpg = null;
			//
			final int length = ins != null ? ins.length : 0;
			//
			Number year, month, day = null;
			//
			List<JapaneseEra> jes = null;
			//
			for (int i = 0; ins != null && i < length; i++) {
				//
				if ((in = ins[i]) == null) {
					//
					continue;
					//
				} // if
					//
				if ((sipush = cast(SIPUSH.class, in)) != null && length > i + 5) {
					//
					year = sipush.getValue();
					//
					month = getNumberValue(ins[i + 1]);
					//
					day = getNumberValue(ins[i + 2]);
					//
					if ((putstatic = cast(PUTSTATIC.class, ins[i + 5])) != null) {
						//
						if (map == null) {
							//
							map = new LinkedHashMap<>();
							//
						} // if
							//
						final String name = putstatic.getFieldName(
								cpg = ObjectUtils.getIfNull(cpg, () -> new ConstantPoolGen(jc.getConstantPool())));
						//
						if ((jes = Arrays.stream(JapaneseEra.values())
								.filter(x -> x != null && StringUtils.equalsIgnoreCase(x.toString(), name))
								.toList()) != null && jes.size() > 1) {
							//
							throw new IllegalStateException();
							//
						} // if
							//
						if (jes == null || jes.isEmpty()) {
							//
							continue;
							//
						} // if
							//
						map.put(jes.get(0),
								year != null && month != null && day != null
										? LocalDate.of(year.intValue(), month.intValue(), day.intValue())
										: null);
						//
					} // if
						//
				} // if
					//
			} // for
				//
		} catch (final IOException e) {
			//
			throw new RuntimeException(e);
			//
		} // try
			//
		return map;
		//
	}

	private static Number getNumberValue(final Instruction in) {
		//
		final ICONST iconst = cast(ICONST.class, in);
		//
		if (iconst != null) {
			//
			return iconst.getValue();
			//
		} // if
			//
		final BIPUSH bipush = cast(BIPUSH.class, in);
		// s
		if (bipush != null) {
			//
			return bipush.getValue();
			//
		} // if
			//
		throw new IllegalStateException();
		//
	}

	private static <T> T cast(final Class<T> clz, final Object instance) {
		return clz != null && clz.isInstance(instance) ? clz.cast(instance) : null;
	}

}

Required dependency
https://mvnrepository.com/artifact/org.apache.bcel/bcel

<!-- https://mvnrepository.com/artifact/org.apache.bcel/bcel -->
<dependency>
	<groupId>org.apache.bcel</groupId>
	<artifactId>bcel</artifactId>
	<version>6.8.2</version>
</dependency>