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 javax.* package in JRE 1.8.0_191 rt.jar. Java source codes are also provided.

⏎ javax/management/relation/RelationType.java

/*
 * Copyright (c) 2000, 2005, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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package javax.management.relation;

import java.util.ArrayList; // for Javadoc
import java.util.List;

import java.io.Serializable;

/**
 * The RelationType interface has to be implemented by any class expected to
 * represent a relation type.
 *
 * @since 1.5
 */
public interface RelationType extends Serializable {

    //
    // Accessors
    //

    /**
     * Returns the relation type name.
     *
     * @return the relation type name.
     */
    public String getRelationTypeName();

    /**
     * Returns the list of role definitions (ArrayList of RoleInfo objects).
     *
     * @return an {@link ArrayList} of {@link RoleInfo}.
     */
    public List<RoleInfo> getRoleInfos();

    /**
     * Returns the role info (RoleInfo object) for the given role info name
     * (null if not found).
     *
     * @param roleInfoName  role info name
     *
     * @return RoleInfo object providing role definition
     * does not exist
     *
     * @exception IllegalArgumentException  if null parameter
     * @exception RoleInfoNotFoundException  if no role info with that name in
     * relation type.
     */
    public RoleInfo getRoleInfo(String roleInfoName)
        throws IllegalArgumentException,
               RoleInfoNotFoundException;
}

⏎ javax/management/relation/RelationType.java

Or download all of them as a single archive file:

File name: jre-rt-javax-1.8.0_191-src.zip
File size: 5381005 bytes
Release date: 2018-10-28
Download 

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

⇐ JRE 8 rt.jar - java.* Package Source Code

⇑ Download and Use JDK 8

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