A Java Swing Layout Manager that keep the aspect ratio of the component inside the container while enlarging/shrinling it. Contains a main() method with a JFrame to see by yourself. I crerated it because was unable to find simple exemples on the net.

SingleComponentAspectRatioKeeperLayout.java

import java.awt.Color;
import java.awt.Component;
import java.awt.Container;
import java.awt.Dimension;
import java.awt.Insets;
import java.awt.LayoutManager;

import javax.swing.JFrame;
import javax.swing.JPanel;

//@Slf4j
/**
 * A Swing Layout that will shrink or enlarge keep the content of a container while keeping
 * it's aspect ratio. The caveat is that only a single component is supported or an exception
 * will be thrown.
 * This is the component's getPreferredSize() method that must return the correct ratio. The
 * preferredSize will not be preserved but the ratio will.
 * @author @francoismarot
 * @see https://gist.github.com/fmarot/f04346d0e989baef1f56ffd83bbf764d
 */
public class SingleComponentAspectRatioKeeperLayout implements LayoutManager {

	/** Will be used for calculus in case no real component is in the parent */
	private static Component fakeComponent = new JPanel();

	public SingleComponentAspectRatioKeeperLayout() {
		fakeComponent.setPreferredSize(new Dimension(0, 0));
	}

	@Override
	public void addLayoutComponent(String arg0, Component arg1) {
	}

	@Override
	public void layoutContainer(Container parent) {
		Component component = getSingleComponent(parent);
		Insets insets = parent.getInsets();
		int maxWidth = parent.getWidth() - (insets.left + insets.right);
		int maxHeight = parent.getHeight() - (insets.top + insets.bottom);

		Dimension prefferedSize = component.getPreferredSize();
		Dimension targetDim = getScaledDimension(prefferedSize, new Dimension(maxWidth, maxHeight));

		double targetWidth = targetDim.getWidth();
		double targetHeight = targetDim.getHeight();

		double hgap = (maxWidth - targetWidth) / 2;
		double vgap = (maxHeight - targetHeight) / 2;

		// Set the single component's size and position.
		component.setBounds((int) hgap, (int) vgap, (int) targetWidth, (int) targetHeight);
	}

	private Component getSingleComponent(Container parent) {
		int parentComponentCount = parent.getComponentCount();
		if (parentComponentCount > 1) {
			throw new IllegalArgumentException(this.getClass().getSimpleName()
					+ " can not handle more than one component");
		}
		Component comp = (parentComponentCount == 1) ? parent.getComponent(0) : fakeComponent;
		return comp;
	}

	private Dimension getScaledDimension(Dimension imageSize, Dimension boundary) {
		double widthRatio = boundary.getWidth() / imageSize.getWidth();
		double heightRatio = boundary.getHeight() / imageSize.getHeight();
		double ratio = Math.min(widthRatio, heightRatio);
		return new Dimension((int) (imageSize.width * ratio), (int) (imageSize.height * ratio));
	}

	@Override
	public Dimension minimumLayoutSize(Container parent) {
		return preferredLayoutSize(parent);
	}

	@Override
	public Dimension preferredLayoutSize(Container parent) {
		return getSingleComponent(parent).getPreferredSize();
	}

	@Override
	public void removeLayoutComponent(Component parent) {
	}

	public static void main(String[] args) {
		JFrame frame = new JFrame();
		JPanel panel = new JPanel(); // the panel we want to keep it's aspect ratio
		panel.setPreferredSize(new Dimension(300, 600));
		panel.setBackground(Color.ORANGE);

		JPanel wrapperPanel = new JPanel(new SingleComponentAspectRatioKeeperLayout());
		wrapperPanel.add(panel);

		frame.getContentPane().add(wrapperPanel);
		frame.setSize(450, 450);

		frame.setVisible(true);
	}
}