3.深刻jvm内核-原理、诊断与优化-10. jvm字节码执行

1、字节码

javap
简单的字节码执行过程
经常使用的字节码
使用ASM生成Java字节码
JIT及其相关参数

1. javap(class文件反汇编工具)

   public class Calc {
   		public int calc() {
   			int a = 500;
   			int b = 200;
   			int c = 50;
   			return (a + b) / c;
   		}
   	}

   javap –verbose Calc

2. 简单的字节码执行过程

3. 经常使用的字节码

4. 经常使用的字节码

5. 使用ASM生成Java字节码

   例子：

   <dependency>
           <groupId>org.ow2.asm</groupId>
           <artifactId>asm</artifactId>
           <version>5.0.3</version>
       </dependency>

   GrandParent.java

   public class GrandParent {
   		public void test() {
   				System.out.println("test of GrandParent");
   		}
   	}

   Parent.java

   public class Parent extends GrandParent{
   		public void test() {
   			System.out.println("test of Parent");
   		}
   	}

   Son.java

   public class Son extends Parent{
   		public void test() {
   			System.out.println("test of Son");
   		}
   	}

   ASMByteCodeManipulation.java

   /**
   	 * description:
   	 *
   	 * [@author](https://my.oschina.net/arthor): dawn.he QQ:       905845006
   	 * [@email](https://my.oschina.net/u/159820): dawn.he@cloudwise.com
   	 * [@email](https://my.oschina.net/u/159820): 905845006@qq.com
   	 * @date: 2019/9/30    9:21 PM
   	 */
   
   	import java.io.FileOutputStream;
   
   	import org.objectweb.asm.ClassWriter;
   	import org.objectweb.asm.MethodVisitor;
   	import org.objectweb.asm.Opcodes;
   
   	public class ASMByteCodeManipulation extends ClassLoader implements Opcodes {
   
   		public static void main(String args[]) throws Exception {
   
   			test1();
   	//        test2();
   		}
   
   		//print hello word
   		public static void test1() throws Exception{
   			ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_MAXS|ClassWriter.COMPUTE_FRAMES);
   			cw.visit(V1_7, ACC_PUBLIC, "Example", null, "java/lang/Object", null);
   			MethodVisitor mw = cw.visitMethod(ACC_PUBLIC, "<init>", "()V", null,  null);
   			mw.visitVarInsn(ALOAD, 0);  //this 入栈
   			//使用invokespecial这种方式也有局限，只能从子类调用。不然报错：
   			mw.visitMethodInsn(INVOKESPECIAL, "java/lang/Object", "<init>", "()V");
   			mw.visitInsn(RETURN);
   			mw.visitMaxs(0, 0);
   			mw.visitEnd();
   			mw = cw.visitMethod(ACC_PUBLIC + ACC_STATIC, "main",  "([Ljava/lang/String;)V", null, null);
   			mw.visitFieldInsn(GETSTATIC, "java/lang/System", "out",  "Ljava/io/PrintStream;");
   			mw.visitLdcInsn("Hello world!");
   			mw.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println",  "(Ljava/lang/String;)V");
   			mw.visitInsn(RETURN);
   			mw.visitMaxs(0,0);
   			mw.visitEnd();
   			byte[] code = cw.toByteArray();
   			ASMByteCodeManipulation loader = new ASMByteCodeManipulation();
   			Class exampleClass = loader
   					.defineClass("Example", code, 0, code.length);
   			exampleClass.getMethods()[0].invoke(null, new Object[] { null });
   
   		}
   		//print hello word
   		public static void test2() throws Exception{
   					ClassWriter cw = new ClassWriter(0);
   			cw.visit(V1_1, ACC_PUBLIC, "Example", null, "Son", null);
   
   			// creates a MethodWriter for the (implicit) constructor
   			MethodVisitor mw = cw.visitMethod(ACC_PUBLIC, "<init>", "()V", null, null);
   			mw.visitVarInsn(ALOAD, 0);
   			mw.visitMethodInsn(INVOKESPECIAL, "Son", "<init>", "()V");
   			mw.visitInsn(RETURN);
   			mw.visitMaxs(1, 1);
   			mw.visitEnd();
   
   			// creates a MethodWriter for the 'test' method
   			mw = cw.visitMethod(ACC_PUBLIC, "test", "()V", null, null);
   			mw.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
   			mw.visitLdcInsn("test of AI3");
   			mw.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println",
   					"(Ljava/lang/String;)V");
   			//Call test() of GrandParent
   			mw.visitVarInsn(ALOAD, 0);
   			mw.visitMethodInsn(INVOKESPECIAL, "GrandParent", "test", "()V");
   			//Call test() of GrandParent
   			mw.visitVarInsn(ALOAD, 0);
   			mw.visitMethodInsn(INVOKESPECIAL, "Parent", "test", "()V");
   			//Call test() of GrandParent
   			mw.visitVarInsn(ALOAD, 0);
   			mw.visitMethodInsn(INVOKESPECIAL, "Son", "test", "()V");
   			mw.visitInsn(RETURN);
   			mw.visitMaxs(2, 1);
   			mw.visitEnd();
   
   			byte[] code = cw.toByteArray();
   			FileOutputStream fos = new FileOutputStream("Example.class");
   			fos.write(code);
   			fos.close();
   
   			ASMByteCodeManipulation loader = new ASMByteCodeManipulation();
   			Class<?> exampleClass = loader.defineClass("Example", code, 0,
   					code.length);
   			Object obj = exampleClass.newInstance();
   			exampleClass.getMethod("test", null).invoke(obj, null);
   
   		}
   
   	}

   test2方法生成的class文件

   public class Example extends Son {
   		public Example() {
   		}
   
   		public void test() {
   			System.out.println("test of AI3");
   			super.test();
   			super.test();
   			super.test();
   		}
   	}

   例子2：

   Account.java

   public class Account {
   		public void operation() {
   			System.out.println("operation....");
   		}
   	}

   要嵌入的内容

   SecurityChecker.java

   public class SecurityChecker { 
   	 public static boolean checkSecurity() { 
   	 System.out.println("SecurityChecker.checkSecurity ...");
   	 return true;
   	 } 
   	}

   AddSecurityCheckClassAdapter.java

   import org.objectweb.asm.ClassVisitor;
   	import org.objectweb.asm.MethodVisitor;
   	import org.objectweb.asm.Opcodes;
   
   	/**
   	 * description:
   	 *
   	 * @author: dawn.he QQ:       905845006
   	 * @email: dawn.he@cloudwise.com
   	 * @email: 905845006@qq.com
   	 * @date: 2019/9/30    10:39 PM
   	 */
   	public class AddSecurityCheckClassAdapter extends ClassVisitor {
   		public AddSecurityCheckClassAdapter( ClassVisitor cv) {
   			super(Opcodes.ASM5, cv);
   		}
   		// 重写 visitMethod，访问到 "operation" 方法时，
   		// 给出自定义 MethodVisitor，实际改写方法内容
   		public MethodVisitor visitMethod(final int access, final String name,
   										 final String desc, final String signature, final String[] exceptions) {
   			MethodVisitor mv = cv.visitMethod(access, name, desc, signature,exceptions);
   			MethodVisitor wrappedMv = mv;
   			if (mv != null) {
   				// 对于 "operation" 方法
   				if (name.equals("operation")) {
   					// 使用自定义 MethodVisitor，实际改写方法内容
   					wrappedMv = new AddSecurityCheckMethodAdapter(mv);
   				}
   			}
   			return wrappedMv;
   		}
   	}

   AddSecurityCheckMethodAdapter.java

   import org.objectweb.asm.MethodVisitor;
   	import org.objectweb.asm.Opcodes;
   
   	/**
   	 * description:
   	 *
   	 * @author: dawn.he QQ:       905845006
   	 * @email: dawn.he@cloudwise.com
   	 * @email: 905845006@qq.com
   	 * @date: 2019/9/30    10:40 PM
   	 */
   	public class AddSecurityCheckMethodAdapter extends MethodVisitor {
   		public AddSecurityCheckMethodAdapter(MethodVisitor mv) {
   			super(Opcodes.ASM5,mv);
   		}
   		public void visitCode() {
   	//        visitMethodInsn(Opcodes.INVOKESTATIC, "geym/jvm/ch10/asm/SecurityChecker",
   			visitMethodInsn(Opcodes.INVOKESTATIC, "SecurityChecker",
   
   					"checkSecurity", "()Z");
   			super.visitCode();
   		}
   	}

   Generator.java

   import org.objectweb.asm.ClassReader;
   	import org.objectweb.asm.ClassWriter;
   	import org.objectweb.asm.Opcodes;
   
   	import java.io.File;
   	import java.io.FileOutputStream;
   
   	/**
   	 * description:
   	 *
   	 * @author: dawn.he QQ:       905845006
   	 * @email: dawn.he@cloudwise.com
   	 * @email: 905845006@qq.com
   	 * @date: 2019/9/30    10:40 PM
   	 */
   	public class Generator extends ClassLoader  {
   		public static void main(String args[]) throws Exception {
   	//        ClassReader cr = new ClassReader("geym.jvm.ch10.asm.Account");
   			ClassReader cr = new ClassReader("Account");
   
   			ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_MAXS|ClassWriter.COMPUTE_FRAMES);
   			AddSecurityCheckClassAdapter classAdapter = new AddSecurityCheckClassAdapter(cw);
   			cr.accept(classAdapter, ClassReader.SKIP_DEBUG);
   			byte[] data = cw.toByteArray();
   
   			//class文件写到哪里
   	//        File file = new File("bin/geym/jvm/ch10/asm/Account.class");
   			File file = new File("Account.class");
   			FileOutputStream fout = new FileOutputStream(file);
   			fout.write(data);
   			fout.close();
   
   
   			Generator loader = new Generator();
   			Class<?> exampleClass = loader.defineClass("Account", data, 0,
   					data.length);
   			Object obj = exampleClass.newInstance();
   			exampleClass.getMethod("operation", null).invoke(obj, null);
   
   		}
   	}

   执行完main方法 输出：

   SecurityChecker.checkSecurity ... operation....

   生成的class

   Account.class

   //
   	// Source code recreated from a .class file by IntelliJ IDEA
   	// (powered by Fernflower decompiler)
   	//
   
   	public class Account {
   		public Account() {
   		}
   
   		public void operation() {
   			SecurityChecker.checkSecurity();
   			System.out.println("operation....");
   		}
   	}

6. JIT及其相关参数

   字节码执行性能较差，因此能够对于热点代码编译成机器码再执行，在运行时的编译，
    叫作JIT Just-In-Time
    JIT的基本思路是，将热点代码，就是执行比较频繁的代码，编译成机器码。