Try catch finally implementation mechanism

try catch finally How to execute jump in case of exception ？finally Why statements must execute ？

The reasons behind it are worth investigating , We from JVM From the perspective of try catch finally The implementation principle behind this syntax .

JVM How to execute try - catch

Create a TryCatchFinallyDemo.java class , stay foo() Method try-catch block; The statement handleException This empty method .

Code example

Use javac The directive compiles it into class file , And use javap -c -v -s View results . Relative to no try-catch block Code for , There is one more in the following code Exception Table.

Compiled instructions

In the compiled bytecode , Each method comes with an exception table （Exception table）, Each row in the exception table represents an exception handler , from from The pointer 、to The pointer 、target The pointer 、 The type of exception caught type form .

The values of these Pointers are bytecode indexes , Used to locate bytecode Its meaning is in [from, to) Bytecode range , An exception of type... Was thrown type It's abnormal , It will jump to target At the bytecode of .

such as , The above example exception table indicates ： stay 0 To 3 middle （ It doesn't contain 3） If it throws Exception abnormal , Jump to 6 perform .

Multiple catch sentence

The following example has more than one catch Examples of statements , Although the following three exceptions will not occur .

Use javac -s You can simply see the corresponding ctach Piece of bytecode .Exception Table Three types of exceptions in , If [0,3) Code segment （ barring 3） Something goes wrong , You can jump to ,6,15,24 Line of code looks for the type of exception that can be caught .

When something goes wrong with the program ,Java The virtual opportunity traverses all the entries in the exception table from top to bottom . When the bytecode index value that triggers the exception is in the... Of an exception entry [from, to) Within the scope of , It will determine whether the exception thrown matches the exception that the entry wants to catch .

• If the match ,Java The virtual opportunity jumps the control flow to target Bytecode to ; If not, continue to traverse the exception table
• If you traverse all the exception tables , Not yet matched to exception handler , Then the The exception will spread to the caller （caller） Repeat the above operation in . In the worst case, the virtual machine needs to traverse the thread Java Exception table for all methods on the stack . If in all callers in the method stack , No matching exception table found ,JVM The current method stack will be cleared .

Multiple exception examples

finally analysis

finally The secret of always executing

that ,JVM How to ensure finally Keywords are always executed ？ We add one to the above example finally block.

finally Code example

Recompile and use javap -c see .

The compiled finally Command section

You can see , The bytecode contains three copies finally Sentence block , The program is normal return And exceptions throw Before . Two of them try and catch call return Before , One is the exception throw Before .

Java The method is replication finally The contents of the code block , We separate try catch All code blocks are normal return and abnormal throw Before . therefore finally Code blocks always execute .

Here are two scenarios that are not commonly used .

finally Modify the return value scenario

The following code runs and the result is 1, still 3 Well ？ This scene can confuse many people . Let's execute on one side , The result is 1.

An example of modifying the return value

Compile and view bytecode ：

Modify the sample bytecode of the return value

By bytecode , We found that , stay try Of the statement return In block ,return The returned variables are not directly returned i value , It's execution finally Block before i Values are stored in the staging area , When executed return The value in the temporary area directly returned by , Even in finally In the statement, the variable i The value of has been modified , It does not affect the returned value .

finally There is return Scene

When finally There is return When the sentence is ,return Statement will rewrite try-block, catch-block The return value of .

Slightly modify the example in the previous chapter ： stay finally Statement to add a line of return value . The result of the operation is 3, Back to finally block The value in .

finally There is return Scene

View bytecode after compilation , And make a comparison with the examples in the previous chapter . On the left is the bytecode compiled from the previous chapter , On the right is the bytecode compiled by the above example .

Every try block, catch block Rear side ,return Before the order , Will be copied finally block Code block for . You can see , although try-catch block Medium i Values are temporarily stored , But because of finally Yes return sentence , Back again finally The modified i value .

finally There is return Scene compiled instructions

summary

• First of all ,JVM Use exception table to handle try-catch Jump logic of ;
• second ,finally Is implemented by copying finally Statement block finally Semantic logic that must be executed ;
• Third , Explained in finally There is return Statements or Throw an abnormal situation .