How JVM handles exceptions

Tue, 21 Jan 2025 14:45:12 +0000

It’s interesting to know how the JVM runs bytecode instructions… But do you know what is going on when an exception is thrown? How does the JVM handle the delegation of control? What does it look like in the bytecode?

Let me quickly show you!

Example

Here’s a simple bit of Java code that includes all the important actors in the exception-throwing game (try-catch-finally):

int a = 0;
try {
 if (a == 0) { // to make it less boring, let's have some branching logic
 throw new Exception("Exception message!");
 }
} catch (Exception e) {
 doSomethingOnException();
} finally {
 doSomethingFinally();
}

Exception table

The resulting bytecode includes an interesting section in the Code attribute called the Exception table. Each method can have its own exception table, and it’s only present when relevant. If there is no exception-handling logic in the method, it won’t have an exception table.

Nataliia Dziubenko on foojay.io - Friends Of OpenJDK

How JVM handles exceptions

Example

Exception table