期望通過註解的形式,給方法套上try catch。老樣子,先看一下try catch的字節碼。
CatchUtil
public class CatchUtil {
private void handle() {
try {
Date date = new Date();
} catch (Exception e) {
e.printStackTrace();
}
}
}
找到編譯後的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
重點就是TRYCATCHBLOCK L0 L1 L2 java/lang/Exception
。label L0
是try塊的起始點,label L1
是終點。方法正常調用不出異常的情況下try塊走完GOTO
到L3,return結束方法;發生異常到異常處理處L2
,將異常加入局部變量表ASTORE
,然後ALOAD
異常,調用catch塊中printStackTrace()打印堆棧。
所以我們需要在方法開始時插入TRYCATCHBLOCK
,異常處理標籤L2後插入printStackTrace()
。
先弄個註解Catch
,只插入註解方法。
@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()
}
}
}
}
重寫visitMaxs()
方法,在此之前插入catch塊。
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塊結束標籤end
,異常處理標籤catch
,然後打印堆棧。
看起來很美好,跑個測試用例。
@Catch
private fun catchVoid() {
val date = Date()
}
@Catch
private fun catchObject() = Date()
找到transform中處理過的class文件,反編譯成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問題了,插入try catch後方法結尾沒有返回值報錯。這裏爲了方便在catch塊中將異常throw出去,當然也可以給方法返回一個默認值。
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)
}
簡單的加個throw拋出異常,再跑一下測試用例。
@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;
}
}
好起來了,個人感覺在catch塊中處理異常後繼續throw出去比較好,畢竟崩潰後定位問題也比較容易。返回默認值雖然能保證不崩潰,但業務邏輯可能產生其它奇奇怪怪的問題。