Javassist是一個開源的分析、編輯和建立Java字節碼的類庫。是由東京工業大學的數學和計算機科學系的 Shigeru Chiba (千葉 滋)所建立的。它已加入了開放源代碼JBoss 應用服務器項目,經過使用Javassist對字節碼操做爲JBoss實現動態"AOP"框架。它能夠用來檢查、」動態」修改以及建立 Java類。其功能與jdk自帶的反射功能相似,但比反射功能更強大。java
package com.javassis.test; public class CoolGuy { private String hotGirlfriendName; private String niceCarModel; public String getHotGirlfriendName() { return hotGirlfriendName; } public void setHotGirlfriendName(String hotGirlfriendName) { this.hotGirlfriendName = hotGirlfriendName; } public String getNiceCarModel() { return niceCarModel; } public void setNiceCarModel(String niceCarModel) { this.niceCarModel = niceCarModel; } } public class Person { private int age; public int getAge() { return age; } public void setAge(int age) { this.age = age; } }
package com.javassis.test; import java.io.IOException; import java.lang.reflect.InvocationTargetException; import java.lang.reflect.Method; import javassist.CannotCompileException; import javassist.ClassPool; import javassist.CtClass; import javassist.CtField; import javassist.CtMethod; import javassist.CtNewMethod; import javassist.Loader; import javassist.NotFoundException; public class Main { public static void main(String[] args) throws NotFoundException, NoSuchMethodException, SecurityException, InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException, CannotCompileException, IOException, ClassNotFoundException { ClassPool classPool = ClassPool.getDefault(); /** * CoolGuy是已經存在的類 */ CtClass ctc = classPool.get("com.javassis.test.CoolGuy"); Class<?> c = ctc.toClass(); Object cg = (Object) c.newInstance(); Method m = c.getDeclaredMethod("setHotGirlfriendName", String.class); m.invoke(cg, "柳巖"); System.out.println("CoolGuy's girlfriend is " + ((CoolGuy) cg).getHotGirlfriendName()); /** * 動態構造一個SuckGuy類 */ CtClass suckCt = classPool.makeClass("com.javassis.test.SuckGuy"); // 添加域 CtField girlfriendField = new CtField(classPool.get("java.lang.String"), "hotGirlfriendName", suckCt); CtField carField = new CtField(classPool.get("java.lang.String"), "niceCarModel", suckCt); suckCt.addField(girlfriendField); suckCt.addField(carField); // 添加方法 CtMethod getMethod = CtNewMethod.make("public String getHotGirlfriendName() { return this.hotGirlfriendName;}", suckCt); CtMethod setMethod = CtNewMethod .make("public void setHotGirlfriendName(String girl) { this.hotGirlfriendName = girl;}", suckCt); suckCt.addMethod(getMethod); suckCt.addMethod(setMethod); getMethod = CtNewMethod.make("public String getNiceCarModel() { return this.niceCarModel;}", suckCt); setMethod = CtNewMethod .make("public void setNiceCarModel(String niceCarModel) { this.niceCarModel = niceCarModel;}", suckCt); suckCt.addMethod(getMethod); suckCt.addMethod(setMethod); Class<?> cuckClass = suckCt.toClass(); Object suckGuy = cuckClass.newInstance(); m = cuckClass.getDeclaredMethod("setHotGirlfriendName", String.class); m.invoke(suckGuy, "鳳姐"); String suckGirlfriend = (String) cuckClass.getMethod("getHotGirlfriendName", null).invoke(suckGuy, null); System.out.println("SuckGuy's girlfriend is " + suckGirlfriend); /** * 設置CoolGuy父類爲Person */ //ctc.writeFile(); ctc.defrost(); // 先解凍,否則會報class is frozen異常, // CtClass對象經過writeFile()、toClass()、toBytecode()轉化爲Class後,Javassist凍結了CtClass對象,所以,JVM不容許再次加載Class文件,因此不容許對其修改 ctc.setSuperclass(classPool.get("com.javassis.test.Person")); Loader cl = new Loader(classPool);// 同一個classloader下不能加載兩次class,因此新new一個loader // c = ctc.toClass(); c = cl.loadClass("com.javassis.test.CoolGuy"); cg = (Object) c.newInstance(); m = c.getMethod("setAge", int.class); m.invoke(cg, 25); m = c.getMethod("getAge", null); System.out.println("CoolGuy's age is " + m.invoke(cg, null)); } }
輸出:服務器
CoolGuy's girlfriend is 柳巖
SuckGuy's girlfriend is 鳳姐
CoolGuy's age is 25app
幾個須要注意的地方:框架
一、筆者用的Javassist版本是3.22.0-GA,比較坑爹的是它必須使用java 9,筆者一開始用java 8會報java.lang.NoClassDefFoundError: java/lang/StackWalker$Optionthis
二、CtClass對象經過writeFile()、toClass()、toBytecode()轉化爲Class後,Javassist凍結了CtClass對象,所以,JVM不容許再次加載Class文件,因此不容許對其修改spa
,再次對其修改時須要defrost() 先解凍,否則會報class is frozen異常。開放源代碼
三、解凍後對類進行修改,以後使用修改後的類須要再次加載,此時雖然已經通過defrost() ,但同一個classloader下不能加載兩次class,再次使用toClass()會報錯,因此新new一個loader,使用loader.loadClass()完成Class加載。code
接下來看看對如何動態的去修改類的方法:對象
思路就是把須要修改的方法改個新名字,而後新建一個新方法,名字命名爲原名字,新方法copy原方法的內容並加入新的代碼。blog
package com.javassis.test; import javassist.ClassPool; import javassist.CtClass; import javassist.CtMethod; import javassist.CtNewMethod; /* * 動態修改CoolGuy的一個方法 * */ public class UpdateTest { public static void main(String[] args) throws Exception { ClassPool classPool = ClassPool.getDefault(); CtClass ctc = classPool.get("com.javassis.test.CoolGuy"); String mName = "getHotGirlfriendName"; CtMethod m = ctc.getDeclaredMethod(mName); m.setName(mName + "$old"); CtMethod nm = CtNewMethod.copy(m, mName, ctc, null); StringBuffer mbody = new StringBuffer(); mbody.append("{"); mbody.append("\n return hotGirlfriendName"); mbody.append(" + \"和鞠婧禕\"; \n"); mbody.append("}"); nm.setBody(mbody.toString()); ctc.addMethod(nm); Class<?> c = ctc.toClass(); CoolGuy coolGuy = (CoolGuy)c.newInstance(); coolGuy.setHotGirlfriendName("柳巖"); System.out.println("CoolGuy's girlfriend is " + coolGuy.getHotGirlfriendName()); } }
運行結果:
CoolGuy's girlfriend is 柳巖和鞠婧禕