Hey there, sorry to hear you're stuck with this super frustrating issue—losing syntax highlighting and code completion makes Java development feel like trying to code in a plain text editor, which is no fun at all. You’ve already tried a bunch of common fixes, so let’s dive into some less obvious but effective steps to get IDEA back on track:

1. Double-Check Project SDK Configuration #

IDEA relies entirely on the Java SDK to provide language-specific features, so a misconfigured SDK is a common hidden culprit:

• Open File > Project Structure (shortcut: Ctrl+Alt+Shift+S on Windows/Linux, Cmd+; on macOS)
• Under Project Settings > Project, make sure Project SDK is set to a valid Java installation (not <No SDK>)
• Verify the Project language level matches your SDK version (e.g., Java 17 should use language level 17)
• Head to Modules > Sources and double-check your src folder is still marked as a Sources root (blue folder icon)—sometimes project imports or config resets can unmark this without warning

2. Invalidate Caches and Restart IDEA #

Corrupted cache files are a frequent cause of weird IDE glitches, and a fresh install won’t clear these since they’re stored in your user directory, not the IDE’s install folder:

• Go to File > Invalidate Caches...
• Select Invalidate and Restart (this will wipe all cached data and rebuild project indexes from scratch)
• Once IDEA restarts, give it a minute to finish reindexing—this often brings back syntax highlighting if cache corruption was the problem

3. Fix File Type Associations #

Sometimes IDEA accidentally marks .java files as plain text, which disables all language-specific tools:

• Open Settings > Editor > File Types (shortcut: Ctrl+Alt+S on Windows/Linux, Cmd+, on macOS)
• First, check the Text category—if *.java is listed there, select it and click the minus (-) button to remove it
• Then, find the Java category and confirm *.java is in its list of associated file patterns (add it with the plus (+) button if it’s missing)

4. Check if the Project is Marked as "Plain Text" #

Occasionally, IDEA might classify an entire project as plain text, which turns off all language support entirely:

• Look at the bottom-right corner of your IDE window—if you see "Plain Text" displayed there, click on it
• From the dropdown menu, select Java to reclassify the project and re-enable Java-specific features

5. Verify IDE Cache Directory Permissions #

If IDEA doesn’t have write access to its cache directory, it can’t build or maintain project indexes properly:

• For Windows: Navigate to C:\Users\<YourUsername>\.IntelliJIdeaCE
• For macOS: ~/Library/Caches/IntelliJIdeaCE
• For Linux: ~/.IntelliJIdeaCE
• Ensure your user account has read/write permissions for this directory. If not, adjust permissions or delete the entire directory (IDEA will rebuild it automatically on restart)

6. Test with a Fresh Java Project #

To rule out issues specific to your existing project, create a brand new Java project:

• Go to File > New > Project, select Java, choose your SDK, and create a simple Main.java file
• If syntax highlighting works here, the problem is likely with your original project—try reimporting it, or deleting the .iml and .idea configuration folders (IDEA will regenerate them)

If none of these steps work, double-check your Java SDK installation by running java -version and javac -version in your terminal—if either command fails, your SDK might be corrupted and need a reinstall.

内容的提问来源于stack exchange，提问作者Pyth0n3nthus1ast