检查非空后检测到空指针异常

Question

我对一个不应该为null的变量有一个奇怪的java.lang.NullPointerException.

Object[] params = new Object[10];
if (param1 != null)
    params[0] = param1;

param1作为方法参数给出,它可以为null.通常,当影响param1到params [0]时,它不为null(否则它不会传递if语句).

这是错误(第144行是"params [0] = param1;"):

Exception in thread "Jalen Agent" java.lang.NullPointerException
    at jalen.MethodStats.onMethodEntry(MethodStats.java:144)
    at hanoi.TowersOfHanoi.solveHanoi(TowersOfHanoi.java)
    at hanoi.TowersOfHanoi.main(TowersOfHanoi.java:29)

以下是发生异常的方法的完整代码:

public static void onMethodEntry(int id, String className, String methodName, Object param1, Object param2, Object param3, Object param4, Object param5) {
    synchronized (globalLock) {
        Object[] params = new Object[10];
        if (param1 != null)
            params[0] = param1;
        if (param2 != null)
            params[1] = param2;
        if (param3 != null)
            params[2] = param3;
        if (param4 != null)
            params[3] = param4;
        if (param5 != null)
            params[4] = param5;
        MethodStats.onMethodEntry(id, className, methodName, params);
    }
}

编辑:

为了澄清我的例子,我在以下背景下:

1. 使用ASM检测Java应用程序
2. 使用Java代理时运行新的检测类
3. 代理将使用由检测代码收集的信息来运行某些测量
4. 代理还会收集应用程序的方法参数值.

为此,onMethodEntry在每个方法运行时执行.我有几种具有不同签名的方法.特别是:

public static void onMethodEntry(int id, String className, String methodName, Object[] params) {
    synchronized (globalLock) {
        StringBuilder fullMethodName = new StringBuilder(className).append('.').append(methodName).append('-').append(Thread.currentThread().getId());
        MethodStats.addNewMethod(id, fullMethodName.toString());
        System.out.println(fullMethodName.toString() + " -- " + id);
        for (Object param : params) {
            if (param != null)
                System.out.println("Param: " + param.toString());
        }
        startTimes[depth] = System.nanoTime();
        stack[depth++] = MethodStats.getMethodInfo(id);
    }
}

public static void onMethodEntry(int id, String className, String methodName, Object param1) {
    synchronized (globalLock) {
        Object[] params = new Object[10];
        if (param1 != null)
            params[0] = param1;
        MethodStats.onMethodEntry(id, className, methodName, params);
    }
}

public static void onMethodEntry(int id, String className, String methodName, Object param1, Object param2) {
    synchronized (globalLock) {
        Object[] params = new Object[10];
        if (param1 != null)
            params[0] = param1;
        if (param2 != null)
            params[1] = param2;
        MethodStats.onMethodEntry(id, className, methodName, params);
    }
}

这是我用于检测程序类(即河内塔)的代码:

public void visitCode() {
    mv.visitLdcInsn(new Integer(this.methodID));
    this.visitLdcInsn(this.className);
    this.visitLdcInsn(this.methodName);
    String paramCall = "";
    if (this.numParam > 0) {
        // Load parameters
        for (int i=1; i<=this.numParam; i++) {
            this.visitVarInsn(Opcodes.ALOAD, i);
            paramCall += "Ljava/lang/Object;";
        }
    }
    System.out.println(paramCall);
    mv.visitMethodInsn(Opcodes.INVOKESTATIC,
            "jalen/MethodStats",
            "onMethodEntry",
            "(ILjava/lang/String;Ljava/lang/String;" + paramCall + ")V");
    super.visitCode();
}

Answer 1

首先,误差不上params[0] = param1线,因为堆栈跟踪显示你的代码中输入的onMethodEntry方法.

添加if语句绝对没有意义:如果param1是null,则不会有任何null分配,但params[0]会保留, null因为所有未分配的位置new Object[10]最初都设置为null.

为了解决这个问题,你应该先弄清楚必须在数组中放置哪些对象而不是null参数,然后按如下方式修改条件:

if (param1 != null) {
    params[0] = param1;
} else {
    params[0] = // something else
}