Question

使用ASM库修改字节代码后无法加载类。

这里是身份变换器，我期望得到与字节1相同大小的修改数组，但它要短2倍！（439 vs 278）

    String path = SimpleClass.class.getName().replace(".", "/") + ".class";
    ClassLoader classLoader = SimpleClass.class.getClassLoader();
    InputStream is = classLoader.getResourceAsStream(path);

    byte[] bytes = IOUtils.toByteArray(is);

    ClassReader reader = new ClassReader(bytes);
    ClassWriter writer = new ClassWriter(reader, 0);
    byte[] modified = writer.toByteArray();

没有什么奇妙的，它无法加载。我怀疑标题是截断的，但是第一个字节是相同的两个阵列。

-54，-2，-70

static class ByteClassLoader extends ClassLoader {
    public Class define(String name, byte[] body) {
        return defineClass(name, body, 0, body.length);
    }
}

ByteClassLoader myLoader = new ByteClassLoader();
Class myClass = myLoader.define("Ooo", modified);

失败并显示错误：

java.lang.UnsupportedClassVersionError: Ooo has been compiled by a more  recent version of the Java Runtime (class file version 0.0), this version of the Java Runtime only recognizes class file versions up to 52.0

at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:763)
at java.lang.ClassLoader.defineClass(ClassLoader.java:642)
at PrintTest$ByteClassLoader.define(PrintTest.java:24)
at PrintTest.x(PrintTest.java:48)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:117)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:42)
at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:253)
at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:84)

Answer 1

这些字节对应于0xcafeba，它与Java类文件幻数0xcafebabe匹配。

这里的问题是new ClassWriter(reader, 0)不会做你认为它做的事情;请参阅API documentation：

classReader - 用于读取原始类的ClassReader。它将用于从原始类复制整个常量池，并在适用的情况下复制原始字节码的其他片段。

我们仍然需要让作家通过ClassReader.accept访问，如下所示：

reader.accept(writer, 0);

作为旁注，您不需要IOUtils.toByteArary，因为ClassReader有constructor taking InputStream。

如何使用ASM（类文件版本0.0）在字节码转换后定义类

1 个答案: