JRE 8 rt.jar is the JAR file for JRE 8 RT (Runtime) libraries. JRE (Java Runtime) 8 is the runtime environment included in JDK 8. JRE 8 rt.jar libraries are divided into 6 packages:

com.* - Internal Oracle and Sun Microsystems libraries
java.* - Standard Java API libraries.
javax.* - Extended Java API libraries.
jdk.* -  JDK supporting libraries.
org.* - Third party libraries.
sun.* - Old libraries developed by Sun Microsystems.

JAR File Information:

Directory of C:\fyicenter\jdk-1.8.0_191\jre\lib
      63,596,151 rt.jar

Here is the list of Java classes of the java.* package in JRE 1.8.0_191 rt.jar. Java source codes are also provided.

⏎ java/lang/invoke/SimpleMethodHandle.java

/*
 * Copyright (c) 2008, 2012, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package java.lang.invoke;

import static java.lang.invoke.LambdaForm.BasicType.*;
import static java.lang.invoke.MethodHandleStatics.*;

/**
 * A method handle whose behavior is determined only by its LambdaForm.
 * @author jrose
 */
final class SimpleMethodHandle extends BoundMethodHandle {
    private SimpleMethodHandle(MethodType type, LambdaForm form) {
        super(type, form);
    }

    /*non-public*/ static BoundMethodHandle make(MethodType type, LambdaForm form) {
        return new SimpleMethodHandle(type, form);
    }

    /*non-public*/ static final SpeciesData SPECIES_DATA = SpeciesData.EMPTY;

    /*non-public*/ public SpeciesData speciesData() {
            return SPECIES_DATA;
    }

    @Override
    /*non-public*/ BoundMethodHandle copyWith(MethodType mt, LambdaForm lf) {
        return make(mt, lf);
    }

    @Override
    String internalProperties() {
        return "\n& Class="+getClass().getSimpleName();
    }

    @Override
    /*non-public*/ public int fieldCount() {
        return 0;
    }

    @Override
    /*non-public*/ final BoundMethodHandle copyWithExtendL(MethodType mt, LambdaForm lf, Object narg) {
        return BoundMethodHandle.bindSingle(mt, lf, narg); // Use known fast path.
    }
    @Override
    /*non-public*/ final BoundMethodHandle copyWithExtendI(MethodType mt, LambdaForm lf, int narg) {
        try {
            return (BoundMethodHandle) SPECIES_DATA.extendWith(I_TYPE).constructor().invokeBasic(mt, lf, narg);
        } catch (Throwable ex) {
            throw uncaughtException(ex);
        }
    }
    @Override
    /*non-public*/ final BoundMethodHandle copyWithExtendJ(MethodType mt, LambdaForm lf, long narg) {
        try {
            return (BoundMethodHandle) SPECIES_DATA.extendWith(J_TYPE).constructor().invokeBasic(mt, lf, narg);
        } catch (Throwable ex) {
            throw uncaughtException(ex);
        }
    }
    @Override
    /*non-public*/ final BoundMethodHandle copyWithExtendF(MethodType mt, LambdaForm lf, float narg) {
        try {
            return (BoundMethodHandle) SPECIES_DATA.extendWith(F_TYPE).constructor().invokeBasic(mt, lf, narg);
        } catch (Throwable ex) {
            throw uncaughtException(ex);
        }
    }
    @Override
    /*non-public*/ final BoundMethodHandle copyWithExtendD(MethodType mt, LambdaForm lf, double narg) {
        try {
            return (BoundMethodHandle) SPECIES_DATA.extendWith(D_TYPE).constructor().invokeBasic(mt, lf, narg);
        } catch (Throwable ex) {
            throw uncaughtException(ex);
        }
    }
}

⏎ java/lang/invoke/SimpleMethodHandle.java

Or download all of them as a single archive file:

File name: jre-rt-java-1.8.0_191-src.zip
File size: 6664831 bytes
Release date: 2018-10-28
Download 

⇒ JRE 8 rt.jar - javax.* Package Source Code

⇐ JRE 8 plugin.jar - Java Deploy Control Panel Plugin

⇑ Download and Use JDK 8

⇑⇑ FAQ for JDK (Java Development Kit)