jvm 如何在使用ASM进行字节码转换后定义类(类文件版本0.0)

9avjhtql  于 2022-11-07  发布在  其他
关注(0)|答案(1)|浏览(146)

使用ASM库修改字节代码后,我无法加载类。
这里是标识转换器,我希望得到修改后的数组,其大小与字节1相同,但它短2倍!(439对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。我怀疑标头被截断,但第一个字节是相同的两个数组。
-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);

失败并出现错误:

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)
lkaoscv7

lkaoscv71#

这些字节对应于0xcafeba,它与Java类文件幻数0xcafebabe相匹配。
这里的问题是new ClassWriter(reader, 0)并没有做你认为它应该做的事情;请参阅API文档:
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。
实际上,我们仍然需要让编写器通过ClassReader.accept * 访问 * 读取器,如下所示:

reader.accept(writer, 0);

顺便说一句,你不需要IOUtils.toByteArary,因为ClassReader有一个接受InputStream的构造函数。

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