所以我对 JVM 内部发生的字节码验证有点困惑。根据Deitel 和Deitel 的书，Java 程序经历了五个阶段(编辑、编译、加载、验证和执行)(第 1 章)。字节码 validator 在“验证”阶段验证字节码。书中没有提到字节码 validator 是类加载器的一部分。

但是根据 docs of oracle ，类加载器执行加载、链接和初始化的任务，在链接过程中它必须验证字节码。

现在，是Deitel和Deitel谈论的字节码验证，以及字节码验证 this oracle document 说说，同样的过程？

或者字节码验证会发生两次，一次在链接过程中，另一次由字节码 validator 进行？

描述了 Dietel 和 Dietel 书中提到的 Java 程序阶段的图片。(我从 nobalG 的以下答案之一中借用了这张图片 :))

最佳答案

您可以通过 Oracle docs 中详细解释的这张图来理解字节码验证。

你会发现字节码验证只发生一次而不是两次

The illustration shows the flow of data and control from Java language source code through the Java compiler, to the class loader and bytecode verifier and hence on to the Java virtual machine, which contains the interpreter and runtime system. The important issue is that the Java class loader and the bytecode verifier make no assumptions about the primary source of the bytecode stream--the code may have come from the local system, or it may have travelled halfway around the planet. The bytecode verifier acts as a sort of gatekeeper: it ensures that code passed to the Java interpreter is in a fit state to be executed and can run without fear of breaking the Java interpreter. Imported code is not allowed to execute by any means until after it has passed the verifier's tests. Once the verifier is done, a number of important properties are known:

• There are no operand stack overflows or underflows
• The types of the parameters of all bytecode instructions are known to always be correct
• Object field accesses are known to be legal--private, public, or protected

While all this checking appears excruciatingly detailed, by the time the bytecode verifier has done its work, the Java interpreter can proceed, knowing that the code will run securely. Knowing these properties makes the Java interpreter much faster, because it doesn't have to check anything. There are no operand type checks and no stack overflow checks. The interpreter can thus function at full speed without compromising reliability.

编辑:-

来自 Oracle 文档 Section 5.3.2 :

When the loadClass method of the class loader L is invoked with the name N of a class or interface C to be loaded, L must perform one of the following two operations in order to load C:

• The class loader L can create an array of bytes representing C as the bytes of a ClassFile structure (§4.1); it then must invoke the method defineClass of class ClassLoader. Invoking defineClass causes the Java Virtual Machine to derive a class or interface denoted by N using L from the array of bytes using the algorithm found in §5.3.5.
• The class loader L can delegate the loading of C to some other class loader L'. This is accomplished by passing the argument N directly or indirectly to an invocation of a method on L' (typically the loadClass method). The result of the invocation is C.

正如 Holger 正确评论的那样，试图在 example 的帮助下进行更多解释:

static int factorial(int n)
{
int res;
for (res = 1; n > 0; n--) res = res * n;
return res;
}

对应的字节码是

method static int factorial(int), 2 registers, 2 stack slots
0: iconst_1 // push the integer constant 1
1: istore_1 // store it in register 1 (the res variable)
2: iload_0 // push register 0 (the n parameter)
3: ifle 14 // if negative or null, go to PC 14
6: iload_1 // push register 1 (res)
7: iload_0 // push register 0 (n)
8: imul // multiply the two integers at top of stack
9: istore_1 // pop result and store it in register 1
10: iinc 0, -1 // decrement register 0 (n) by 1
11: goto 2 // go to PC 2
14: iload_1 // load register 1 (res)
15: ireturn // return its value to caller

请注意，JVM 中的大部分指令都是类型化的。

现在您应该注意，除非代码至少满足以下条件，否则无法保证 JVM 的正常运行:

• 类型正确性:指令的参数总是 指令预期的类型。
• 没有堆栈溢出或下溢:一条指令从不弹出参数 从空栈中取出，也不将结果压入满栈(其大小为 等于为方法声明的最大堆栈大小)。
• 代码包含:程序计数器必须始终指向 该方法的代码，到有效指令编码的开头 (没有落到方法代码的末尾；没有分支到 指令编码的中间)。
• 寄存器初始化:从寄存器加载必须始终跟随在 该寄存器中至少有一个商店；换句话说，寄存器 不对应方法参数未在方法上初始化 入口，从未初始化的寄存器加载是错误的。
• 对象初始化:当一个类 C 的实例被创建时，一个 C类的初始化方法(对应于 此类的构造函数)必须在类之前调用 可以使用实例。

字节码验证的目的是一劳永逸地检查这些条件，通过在加载时对字节码进行静态分析。通过验证的字节码可以更快地执行。

还要注意字节码验证的目的是将上面列出的验证从运行时转移到加载时。

_{以上解释摘自Java bytecode verification: algorithms and formalizations}

关于java - 字节码的验证会发生两次吗？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/25541778/