ASM给方法加try catch

期望通过注解的形式，给方法套上try catch。老样子，先看一下try catch的字节码。

CatchUtil

public class CatchUtil {
    private void handle() {
        try {
            Date date = new Date();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

找到编译后的class文件，使用插件ASM Bytecode Viewer

 private handle()V
    TRYCATCHBLOCK L0 L1 L2 java/lang/Exception
   L0
    LINENUMBER 8 L0
    NEW java/util/Date
    DUP
    INVOKESPECIAL java/util/Date.<init> ()V
    ASTORE 1
   L1
    LINENUMBER 12 L1
    GOTO L3
   L2
    LINENUMBER 9 L2
   FRAME SAME1 java/lang/Exception
    ASTORE 1
   L4
    LINENUMBER 10 L4
    ALOAD 1
    INVOKEVIRTUAL java/lang/Exception.printStackTrace ()V
   L3
    LINENUMBER 13 L3
   FRAME SAME
    RETURN

重点就是TRYCATCHBLOCK L0 L1 L2 java/lang/Exception。label L0是try块的起始点，label L1是终点。方法正常调用不出异常的情况下try块走完GOTO到L3，return结束方法；发生异常到异常处理处L2，将异常加入局部变量表ASTORE，然后ALOAD异常，调用catch块中printStackTrace()打印堆栈。

所以我们需要在方法开始时插入TRYCATCHBLOCK，异常处理标签L2后插入printStackTrace()。

先弄个注解Catch，只插入注解方法。

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD})
public @interface Catch {
}

CatchMethodVisitor在方法前插入TRYCATCHBLOCK。

object CatchMethodVisitor {

    operator fun invoke(
            mv: MethodVisitor,
            access: Int,
            name: String?,
            descriptor: String?,
    ): MethodVisitor {
        return object : AdviceAdapter(Opcodes.ASM9, mv, access, name, descriptor) {
            private var needTrack = false
            private val start = Label()
            private val end = Label()
            private val catch = Label()

            override fun visitAnnotation(descriptor: String?, visible: Boolean): AnnotationVisitor {
                if (Type.getDescriptor(Catch::class.java) == descriptor) needTrack = true
                return super.visitAnnotation(descriptor, visible)
            }

            override fun onMethodEnter() {
                if (needTrack) {
                    visitLabel(start)
                    visitTryCatchBlock(start, end, catch, "java/lang/Exception")
                }
                super.onMethodEnter()
            }
        }
    }
}

重写visitMaxs()方法，在此之前插入catch块。

            override fun visitMaxs(maxStack: Int, maxLocals: Int) {
                if (needTrack) {
                    mv.visitLabel(end)
                    mv.visitLabel(catch)
                    mv.visitFrame(Opcodes.F_SAME1, 0, null, 1, arrayOf<Any>("java/lang/Exception"))
                    val local = newLocal(Type.LONG_TYPE);
                    mv.visitVarInsn(ASTORE, local)
                    mv.visitVarInsn(ALOAD, local)
                    mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Exception", "printStackTrace", "()V", false)
                }
                super.visitMaxs(maxStack, maxLocals)
            }

try块结束标签end，异常处理标签catch，然后打印堆栈。

看起来很美好，跑个测试用例。

    @Catch
    private fun catchVoid() {
        val date = Date()
    }

    @Catch
    private fun catchObject() = Date()

找到transform中处理过的class文件，反编译成Java。

   @Catch
   private final void catchVoid() {
      try {
         new Date();
      } catch (Exception var3) {
         var3.printStackTrace();
      }
   }

   @Catch
   private final Date catchObject() {
      try {
         return new Date();
      } catch (Exception var2) {
         var2.printStackTrace();
      }
   }

嗯哼~原方法中有返回值就有问题了，插入try catch后方法结尾没有返回值报错。这里为了方便在catch块中将异常throw出去，当然也可以给方法返回一个默认值。

            override fun visitMaxs(maxStack: Int, maxLocals: Int) {
                if (needTrack) {
                    mv.visitLabel(end)
                    mv.visitLabel(catch)
                    mv.visitFrame(Opcodes.F_SAME1, 0, null, 1, arrayOf<Any>("java/lang/Exception"))
                    val local = newLocal(Type.LONG_TYPE);
                    mv.visitVarInsn(ASTORE, local)
                    mv.visitVarInsn(ALOAD, local)
                    mv.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Exception", "printStackTrace", "()V", false)
                    mv.visitVarInsn(ALOAD, local)
                    mv.visitInsn(ATHROW)
                }
                super.visitMaxs(maxStack, maxLocals)
            }

简单的加个throw抛出异常，再跑一下测试用例。

   @Catch
   private final void catchVoid() {
      try {
         new Date();
      } catch (Exception var3) {
         var3.printStackTrace();
         throw var3;
      }
   }

   @Catch
   private final Date catchObject() {
      try {
         return new Date();
      } catch (Exception var2) {
         var2.printStackTrace();
         throw var2;
      }
   }

好起来了，个人感觉在catch块中处理异常后继续throw出去比较好，毕竟崩溃后定位问题也比较容易。返回默认值虽然能保证不崩溃，但业务逻辑可能产生其它奇奇怪怪的问题。