小编典典

在使用ASM进行字节代码转换后如何定义类(类文件版本0.0)

java

使用ASM库修改字节码后,无法加载类。

这是身份转换器,我希望得到的修改数组的大小与字节1相同,但要短2倍!(439和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();

加载失败没有什么奇妙的。我怀疑标头被截断了,但是两个数组中的第一个字节都相同。

-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)

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2020-11-23

共1个答案

小编典典

这些字节对应于0xcafeba与Java类文件幻数匹配的字节0xcafebabe

这里的问题是,new ClassWriter(reader, 0)您无法做到自己想像的那样。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。

我们实际上仍然需要作者通过来 访问
读者ClassReader.accept,如下所示:

reader.accept(writer, 0);

附带说明一下,您不需要IOUtils.toByteArary像那样ClassReader构造函数InputStream

2020-11-23