Use bytecode to see try-catch-finally and return

I have read an article aboutReturnAndFinallyThe article on execution sequence is analyzed only at the Java language level. In fact, I think it's easy to seeBytecodeIt may be clearer.

Recently, I have been looking at Java Virtual Machine specifications and found that direct analysis of bytecode can help me better understand the Java language.

First, there is only oneTry-finally, NoCatch.

Try-finally

public class ExceptionTest {  public void tryFinally() {    try {      tryItOut();    } finally {      wrapItUp();    }  }  // auxiliary methods  public void tryItOut() { }  public void wrapItUp() {}}

Passjavap -c ExceptionTestTo view its bytecode.

public void tryFinally();  Code:     0: aload_0     1: invokevirtual #2  // Method tryItOut:()V     4: aload_0     5: invokevirtual #3  // Method wrapItUp:()V     8: goto          18    11: astore_1    12: aload_0    13: invokevirtual #3  // Method wrapItUp:()V    16: aload_1    17: athrow    18: return  Exception table:     from    to  target type         0     4    11   any

If no exception is thrown, the execution sequence is

0: aload_01: invokevirtual #2  // Method tryItOut:()V4: aload_05: invokevirtual #3  // Method wrapItUp:()V18: return

If an exception is thrown, the JVM will

Exception table:   from    to  target type       0     4    11   any

. If the command is between 0 and 4 bytes, it throws any type (Any Type) Will jump to 11 bytes to continue running.

11: astore_112: aload_013: invokevirtual #316: aload_117: athrow

Astore_1Saves the thrown exception objectLocal variableThe second element of the array. The following two lines of commands are used to call the member method.wrapItUp.

12: aload_013: invokevirtual #3

Finally

16: aload_117: athrow

Throw an exception again.

Through the above analysis, we can draw a conclusion

In try-Finally, the exception thrown by the try block is saved in local variable first, and then the Finally block is executed. After the execution is complete, the exception is thrown again.

If we modify the code, return directly in the try block.

Try-return-finally

public void tryFinally() {  try {    tryItOut();    return;  } finally {    wrapItUp();  }}

"Disassembly:

 0: aload_0 1: invokevirtual #2 // Method tryItOut:()V 4: aload_0 5: invokevirtual #3 // Method wrapItUp:()V 8: return 9: astore_110: aload_011: invokevirtual #3 // Method wrapItUp:()V14: aload_115: athrow

It can be seen that the Finally block code is still put before return.

If the try block containsreturn statementThe finally code must be executed first and then returned.

This is the JVM specification.

Compilation ofTry-finallyStatement is similar to thatTry-catch. Pior to transferring control outsideTryStatement, whether that transfer is normal or abrupt, because an exception has been thrown,FinallyClause must first be execute.

Try-catch-finally

Add a catch block to the above Code

public void tryCatchFinally() {  try {    tryItOut();  } catch (TestExc e) {    handleExc(e);  } finally {    wrapItUp();  }}

Javap

public void tryCatchFinally();  Code:     0: aload_0     1: invokevirtual #2     4: aload_0     5: invokevirtual #3     8: goto          31    11: astore_1    12: aload_0    13: aload_1    14: invokevirtual #5                      17: aload_0    18: invokevirtual #3    21: goto          31    24: astore_2    25: aload_0    26: invokevirtual #3    29: aload_2    30: athrow    31: returnException table:   from    to  target type       0     4    11   Class TestExc       0     4    24   any      11    17    24   any

PassException tableWe can see that:

• Catch listening 0 ~ The 4-byte type isTextExc.
• Finally is 0 ~ 4 and 11 ~ 17 bytes exception of any type.

That is to sayCatch BlockIt is also inFinally blockWithin the jurisdiction. IfCatch BlockInReturn Statement, So it must be inFinally blockThen run.

View Original article http://www.liangfeizc.com/blog/article/32/

Use bytecode to see try-catch-finally and return