Js解释器V8引擎嵌入的异常处理

v8引擎为Google开发的JS语言解释器有着高效易用等特点，通常它执行一个Js脚本需要经过编译和运行步骤，

由于我们的脚本程序很可能不正确，随时造成过程环节的异常，我们来解决一个异常以及错误的捕捉和处理过程，如下脚本：

test.js:

var str = 'Hello';
str = str + ', World!';
alert(str);

　　

使用以下经典的V8 C++示例代码来执行脚本：

#include <v8.h>
using namespace v8;

//提取js文件
v8::Handle<v8::String> ReadFile(const char* name)
{
    FILE* file = fopen(name, "rb");
    if (file == NULL) return v8::Handle<v8::String>();
    fseek(file, 0, SEEK_END);
    int size = ftell(file);
    rewind(file);
    char* chars = new char[size + 1];
    chars[size] = '\0';
    for (int i = 0; i < size;)
    {
     int read = fread(&chars[i], 1, size - i, file);
     i += read;
    }
    fclose(file);
    v8::Handle<v8::String> result = v8::String::New(chars, size);
    delete[] chars;
    return result;
}

int main()
{
    HandleScope handle_scope;
    Handle<Context> context = Context::New();
    Context::Scope context_scope(context);

    const char * fname = "test.js";
    Handle<String> source = ReadFile(fname);
    //ExecuteString(source, String::New(fname), true, true);

    Handle<Script> script = Script::Compile(source);
    Handle<Value> result = script->Run();
    String::AsciiValue ascii(result);
    printf("%s\n", *ascii);
    return 0;
}

　

然后编译程序 

# g++ -o test test.cpp -lv8 

# ./test

<unknown>:58: Uncaught ReferenceError: alert is not defined

Segmentation fault

在执行过程遇到JS语法错误时，V8很干脆地中上了进程，提醒段错误。

在这我们正常应用环境是无法使用的，需要有一种异常处理机制来收集并处理错误。

这就是v8::TryCatch ，在编译前声明，在之后的编译和执行中只要有错误的发生均可捕获。

查找了相关的V8源代码，改进后的代码实现如下(函数体)，

其中source为源文件内容，name用来标记当前脚本名称(文件名)，

print_result表示是否打印脚本执行结果，report_exceptions表示是否报告异常(错误)：

　

bool ExecuteString(v8::Handle<v8::String> source,
                   v8::Handle<v8::Value> name,
                   bool print_result,
                   bool report_exceptions)
{
  v8::HandleScope handle_scope;
  v8::TryCatch try_catch;   //这里设置异常机制
  v8::Handle<v8::Script> script = v8::Script::Compile(source, name);
  if (script.IsEmpty()) {
    // Print errors that happened during compilation.
    if (report_exceptions)
      ReportException(&try_catch);
    return false;
  } else {
    v8::Handle<v8::Value> result = script->Run();
    if (result.IsEmpty()) {
      assert(try_catch.HasCaught());
      // Print errors that happened during execution.
      if (report_exceptions)
        ReportException(&try_catch);
      return false;
    } else {
      assert(!try_catch.HasCaught());
      if (print_result && !result->IsUndefined()) {
        // If all went well and the result wasn't undefined then print
        // the returned value.
        v8::String::Utf8Value str(result);
        const char* cstr = ToCString(str);
        printf("%s\n", cstr);
      }
      return true;
    }
  }
}

　

相关的异常处理函数：

　

const char* ToCString(const v8::String::Utf8Value& value) {
  return *value ? *value : "<string conversion failed>";
}


void ReportException(v8::TryCatch* try_catch) {
  v8::HandleScope handle_scope;
  v8::String::Utf8Value exception(try_catch->Exception());
  const char* exception_string = ToCString(exception);
  v8::Handle<v8::Message> message = try_catch->Message();
  if (message.IsEmpty()) {
    // V8 didn't provide any extra information about this error; just
    // print the exception.
    printf("%s\n", exception_string);
  } else {
    // Print (filename):(line number): (message).
    v8::String::Utf8Value filename(message->GetScriptResourceName());
    const char* filename_string = ToCString(filename);
    int linenum = message->GetLineNumber();
    printf("%s:%i: %s\n", filename_string, linenum, exception_string);
    // Print line of source code.
    v8::String::Utf8Value sourceline(message->GetSourceLine());
    const char* sourceline_string = ToCString(sourceline);
    printf("%s\n", sourceline_string);
    // Print wavy underline (GetUnderline is deprecated).
    int start = message->GetStartColumn();
    for (int i = 0; i < start; i++) {
      printf(" ");
    }
    int end = message->GetEndColumn();
    for (int i = start; i < end; i++) {
      printf("^");
    }
    printf("\n");
    v8::String::Utf8Value stack_trace(try_catch->StackTrace());
    if (stack_trace.length() > 0) {
      const char* stack_trace_string = ToCString(stack_trace);
      printf("%s\n", stack_trace_string);
    }
  }
}

　

最后修改main入口：

int main(int argc, char* argv[])
{
    HandleScope handle_scope;

    Handle<Context> context = Context::New();
    Context::Scope context_scope(context);

    const char * fname = "test.js";
    Handle<String> source = ReadFile(fname);
    ExecuteString(source, String::New(fname), true, true);

    return 0;
}

　　

重新编译后运行结果为：

# ./test

test.js:3: ReferenceError: alert is not defined

alert(str);

^

ReferenceError: alert is not defined

    at test.js:3:1

已经可以正常报告错误。