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/beans/ParameterDescriptor.java

/*
 * Copyright (c) 1996, 1997, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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package java.beans;

/**
 * The ParameterDescriptor class allows bean implementors to provide
 * additional information on each of their parameters, beyond the
 * low level type information provided by the java.lang.reflect.Method
 * class.
 * <p>
 * Currently all our state comes from the FeatureDescriptor base class.
 */

public class ParameterDescriptor extends FeatureDescriptor {

    /**
     * Public default constructor.
     */
    public ParameterDescriptor() {
    }

    /**
     * Package private dup constructor.
     * This must isolate the new object from any changes to the old object.
     */
    ParameterDescriptor(ParameterDescriptor old) {
        super(old);
    }

}

⏎ java/beans/ParameterDescriptor.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

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