Java ASM系列：（035）TraceClassVisitor介绍

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本文属于[Java ASM系列一：Core API](https://blog.51cto.com/lsieun/2924583)当中的一篇。 `TraceClassVisitor` class extends the `ClassVisitor` class, and builds **a textual representation of the visited class**. ## 1. TraceClassVisitor类 ### 1.1 class info 第一个部分，`TraceClassVisitor`类继承自`ClassVisitor`类，而且有`final`修饰，因此不会存在子类。 ```java public final class TraceClassVisitor extends ClassVisitor { } ``` ### 1.2 fields 第二个部分，`TraceClassVisitor`类定义的字段有哪些。`TraceClassVisitor`类有两个重要的字段，一个是`PrintWriter printWriter`用于打印；另一个是`Printer p`将class转换成文字信息。 ```java public final class TraceClassVisitor extends ClassVisitor { private final PrintWriter printWriter; // 真正打印输出的类 public final Printer p; // 信息采集器 } ``` ### 1.3 constructors 第三个部分，`TraceClassVisitor`类定义的构造方法有哪些。 ```java public final class TraceClassVisitor extends ClassVisitor { public TraceClassVisitor(final PrintWriter printWriter) { this(null, printWriter); } public TraceClassVisitor(final ClassVisitor classVisitor, final PrintWriter printWriter) { this(classVisitor, new Textifier(), printWriter); } public TraceClassVisitor(final ClassVisitor classVisitor, final Printer printer, final PrintWriter printWriter) { super(Opcodes.ASM10_EXPERIMENTAL, classVisitor); this.printWriter = printWriter; this.p = printer; } } ``` ### 1.4 methods 第四个部分，`TraceClassVisitor`类定义的方法有哪些。对于`TraceClassVisitor`类的`visit()`、`visitField()`、`visitMethod()`和`visitEnd()`方法，会分别调用`Printer.visit()`、`Printer.visitField()`、`Printer.visitMethod()`和`Printer.visitClassEnd()`方法。 ```java public final class TraceClassVisitor extends ClassVisitor { @Override public void visit(final int version, final int access, final String name, final String signature, final String superName, final String[] interfaces) { p.visit(version, access, name, signature, superName, interfaces); super.visit(version, access, name, signature, superName, interfaces); } @Override public FieldVisitor visitField(final int access, final String name, final String descriptor, final String signature, final Object value) { Printer fieldPrinter = p.visitField(access, name, descriptor, signature, value); return new TraceFieldVisitor(super.visitField(access, name, descriptor, signature, value), fieldPrinter); } @Override public MethodVisitor visitMethod(final int access, final String name, final String descriptor, final String signature, final String[] exceptions) { Printer methodPrinter = p.visitMethod(access, name, descriptor, signature, exceptions); return new TraceMethodVisitor(super.visitMethod(access, name, descriptor, signature, exceptions), methodPrinter); } @Override public void visitEnd() { p.visitClassEnd(); if (printWriter != null) { p.print(printWriter); // Printer和PrintWriter进行结合 printWriter.flush(); } super.visitEnd(); } } ``` ## 2. 如何使用TraceClassVisitor类 使用`TraceClassVisitor`类，很重点的一点就是选择`Printer`类的具体实现，可以选择`ASMifier`类，也可以选择`Textifier`类（默认）： ```text boolean flag = true or false; Printer printer = flag ? new ASMifier() : new Textifier(); PrintWriter printWriter = new PrintWriter(System.out, true); TraceClassVisitor traceClassVisitor = new TraceClassVisitor(null, printer, printWriter); ``` ### 2.1 生成新的类 ```java import lsieun.utils.FileUtils; import org.objectweb.asm.ClassWriter; import org.objectweb.asm.FieldVisitor; import org.objectweb.asm.MethodVisitor; import org.objectweb.asm.util.TraceClassVisitor; import java.io.PrintWriter; import static org.objectweb.asm.Opcodes.*; public class TraceClassVisitorExample01Generate { public static void main(String[] args) throws Exception { String relative_path = "sample/HelloWorld.class"; String filepath = FileUtils.getFilePath(relative_path); // (1) 生成byte[]内容 byte[] bytes = dump(); // (2) 保存byte[]到文件 FileUtils.writeBytes(filepath, bytes); } public static byte[] dump() throws Exception { // (1) 创建ClassWriter对象 ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_FRAMES); PrintWriter printWriter = new PrintWriter(System.out); TraceClassVisitor cv = new TraceClassVisitor(cw, printWriter); // (2) 调用visitXxx()方法 cv.visit(V1_8, ACC_PUBLIC + ACC_SUPER, "sample/HelloWorld", null, "java/lang/Object", null); { FieldVisitor fv1 = cv.visitField(ACC_PRIVATE, "intValue", "I", null, null); fv1.visitEnd(); } { FieldVisitor fv2 = cv.visitField(ACC_PRIVATE, "strValue", "Ljava/lang/String;", null, null); fv2.visitEnd(); } { MethodVisitor mv1 = cv.visitMethod(ACC_PUBLIC, "", "()V", null, null); mv1.visitCode(); mv1.visitVarInsn(ALOAD, 0); mv1.visitMethodInsn(INVOKESPECIAL, "java/lang/Object", "", "()V", false); mv1.visitInsn(RETURN); mv1.visitMaxs(0, 0); mv1.visitEnd(); } { MethodVisitor mv2 = cv.visitMethod(ACC_PUBLIC, "test", "()V", null, null); mv2.visitCode(); mv2.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;"); mv2.visitLdcInsn("Hello World"); mv2.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false); mv2.visitInsn(RETURN); mv2.visitMaxs(2, 1); mv2.visitEnd(); } cv.visitEnd(); // (3) 调用toByteArray()方法 return cw.toByteArray(); } } ``` ### 2.2 修改已有的类 ```java import lsieun.utils.FileUtils; import org.objectweb.asm.Cla***eader; import org.objectweb.asm.ClassVisitor; import org.objectweb.asm.ClassWriter; import org.objectweb.asm.Opcodes; import org.objectweb.asm.util.TraceClassVisitor; import java.io.PrintWriter; public class TraceClassVisitorExample02Transform { public static void main(String[] args) { String relative_path = "sample/HelloWorld.class"; String filepath = FileUtils.getFilePath(relative_path); byte[] bytes1 = FileUtils.readBytes(filepath); //（1）构建Cla***eader Cla***eader cr = new Cla***eader(bytes1); //（2）构建ClassWriter ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_FRAMES); //（3）串连ClassVisitor int api = Opcodes.ASM9; PrintWriter printWriter = new PrintWriter(System.out); TraceClassVisitor tcv = new TraceClassVisitor(cw, printWriter); ClassVisitor cv = new MethodTimerVisitor(api, tcv); //（4）结合Cla***eader和ClassVisitor int parsingOptions = Cla***eader.SKIP_DEBUG | Cla***eader.SKIP_FRAMES; cr.accept(cv, parsingOptions); //（5）生成byte[] byte[] bytes2 = cw.toByteArray(); FileUtils.writeBytes(filepath, bytes2); } } ``` ### 2.3 打印ASM代码 ```java import org.objectweb.asm.Cla***eader; import org.objectweb.asm.util.ASMifier; import org.objectweb.asm.util.Printer; import org.objectweb.asm.util.Textifier; import org.objectweb.asm.util.TraceClassVisitor; import java.io.IOException; import java.io.PrintWriter; /** * 这里的代码是参考自{@link org.objectweb.asm.util.Printer#main} */ public class ASMPrint { public static void main(String[] args) throws IOException { // (1) 设置参数 String className = "sample.HelloWorld"; int parsingOptions = Cla***eader.SKIP_FRAMES | Cla***eader.SKIP_DEBUG; boolean asmCode = true; // (2) 打印结果 Printer printer = asmCode ? new ASMifier() : new Textifier(); PrintWriter printWriter = new PrintWriter(System.out, true); TraceClassVisitor traceClassVisitor = new TraceClassVisitor(null, printer, printWriter); new Cla***eader(className).accept(traceClassVisitor, parsingOptions); } } ``` ## 3. 总结 本文对`TraceClassVisitor`类进行了介绍，内容总结如下： - 第一点，从整体上来说，`TraceClassVisitor`类的作用是什么。它能够将class文件的内容转换成文字输出。 - 第二点，从结构上来说，`TraceClassVisitor`类的各个部分包含哪些信息。 - 第三点，从使用上来说，`TraceClassVisitor`类的输出结果依赖于`Printer`类的具体实现，可以选择`ASMifier`类输出ASM代码，也可以选择`Textifier`类输出Instruction信息。

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