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/awt/LayoutManager.java

/*
 * Copyright (c) 1995, 2005, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */
package java.awt;

/**
 * Defines the interface for classes that know how to lay out
 * <code>Container</code>s.
 * <p>
 * Swing's painting architecture assumes the children of a
 * <code>JComponent</code> do not overlap.  If a
 * <code>JComponent</code>'s <code>LayoutManager</code> allows
 * children to overlap, the <code>JComponent</code> must override
 * <code>isOptimizedDrawingEnabled</code> to return false.
 *
 * @see Container
 * @see javax.swing.JComponent#isOptimizedDrawingEnabled
 *
 * @author      Sami Shaio
 * @author      Arthur van Hoff
 */
public interface LayoutManager {
    /**
     * If the layout manager uses a per-component string,
     * adds the component <code>comp</code> to the layout,
     * associating it
     * with the string specified by <code>name</code>.
     *
     * @param name the string to be associated with the component
     * @param comp the component to be added
     */
    void addLayoutComponent(String name, Component comp);

    /**
     * Removes the specified component from the layout.
     * @param comp the component to be removed
     */
    void removeLayoutComponent(Component comp);

    /**
     * Calculates the preferred size dimensions for the specified
     * container, given the components it contains.
     * @param parent the container to be laid out
     *
     * @see #minimumLayoutSize
     */
    Dimension preferredLayoutSize(Container parent);

    /**
     * Calculates the minimum size dimensions for the specified
     * container, given the components it contains.
     * @param parent the component to be laid out
     * @see #preferredLayoutSize
     */
    Dimension minimumLayoutSize(Container parent);

    /**
     * Lays out the specified container.
     * @param parent the container to be laid out
     */
    void layoutContainer(Container parent);
}

⏎ java/awt/LayoutManager.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)