Text Area Scrolling

A JTextArea, added to a scroll pane, is commonly used to display messages generated by an application as it is easy to add messages to the text area using the append(…) method. When using a text area in this manner there is usually also a requirement that the text area scroll as each new message is added. Sometimes the text area scrolls automatically and sometimes it doesn’t. Why is this?

Based on my observations, when using JDK1.4.2, I have noticed that two requirements must be met for automatic scrolling:

• the append(…) must be done on the Event Dispatch Thread
• the caret must currently be positioned at the end of the text area before the append(…) method is invoked

I have also noted that initializing a text area at creation time using any of the following approaches will cause the caret to be positioned at the start of the text area (not the end as might be expected), and therefore scrolling will not happen automatically:

• JTextArea textArea = new JTextArea(“Initial text”, …);
• textArea.setText(“Initial text”);
• textArea.read(someFile, null);

So, in general, automatic scrolling will only work on an initially empty text area when you use the append(…) method that is invoked from the Event Dispatch Thread.

So what can we do when the above conditions are not met?

The most common suggestion you will find in the forums is to use code like the following:

textArea.append(...);
textArea.setCaretPosition(textArea.getDocument().getLength());

Unfortunately, you must reset the caret position with every append(…) to make sure the caret is at the end of the document.

Note: I probably should have titled this entry “Text Component Scrolling” since the suggestions will also apply to JTextPane.

Update September 29, 2015

When I originally wrote this blog I suggested you could use code like the following:

JTextArea textArea = new JTextArea();
DefaultCaret caret = (DefaultCaret)textArea.getCaret();
caret.setUpdatePolicy(DefaultCaret.ALWAYS_UPDATE);

to cause the text area to scroll every time new text was added to the text area. This was wrong and was a misunderstanding of the “setUpdatePolicy(…)” functionality. There appears to be no API shortcut to force scrolling if any of the original 3 conditions are not met.

However, you can check out the Smart Scrolling link below which may provide a helpful scrolling feature.

See Also

Smart Scrolling

Related Reading

The Event Dispatch Thread
Java API: javax.swing.text.DefaultCaret.setUpdatePolicy(…)