通过堆栈信息，可以知道PrintManager.getPrintServices出现了空指针。这里也不用看代码就能猜到，因为我们开机没有启动打印服务，所以肯定get不到这个服务的。

然后考虑修改方案，增加非空保护是不是就可以了？Naive！我们的目的是裁剪打印服务，所以我们的修改点并不在这个服务本身，而是删除所以调用这个服务的地方。

所以堆栈显示的PrintManager，PrintServicesLoader什么的统统不要改，我们要看代码结构，看看是怎么调用进来的。通过阅读代码，了解到系统里有很多Loader类型的对象，其中一个子类就是PrintServicesLoader。然后这些Loader是由LoaderManager管理启动的。而LoaderManager在DynamicIndexableContentMonitor.java出现过一次初始化操作。

看下DynamicIndexableContentMonitor.java代码：

public void register(Activity activity, int loaderId) {
    ...
    boolean hasFeaturePrinting = mContext.getPackageManager().hasSystemFeature(PackageManager.FEATURE_PRINTING);
    ...
    if (hasFeaturePrinting) {
        activity.getLoaderManager().initLoader(loaderId, null, this);
    }
    ...

有木有发现一个熟悉的代码？

对，代码里再次出现了一个有关SystemFeature的判断！上一次出现时SystemServer启动服务前也做了相同的判断。

所以要裁剪掉打印机服务，我们只需要将FEATURE_PRINTING关闭即可。

通过修改SystemFeature判断后，在SystemServer里面的裁剪代码就可以不再添加了。但是有些服务的裁剪Android并没有添加系统特性的处理，所以还是建议使用我的方法进行裁剪。

SystemFeature加载流程

先看一看FEATURE_PRINTING

PackageManager.java

/**
 * Feature for {@link #getSystemAvailableFeatures} and {@link #hasSystemFeature}:
 * The device supports printing.
 */
@SdkConstant(SdkConstantType.FEATURE)
public static final String FEATURE_PRINTING = "android.software.print";

/**
 * Get a list of features that are available on the
 * system.
 *
 * @return An array of FeatureInfo classes describing the features
 * that are available on the system, or null if there are none(!!).
 */
public abstract FeatureInfo[] getSystemAvailableFeatures();

/**
 * Check whether the given feature name is one of the available features as
 * returned by {@link #getSystemAvailableFeatures()}. This tests for the
 * presence of <em>any</em> version of the given feature name; use
 * {@link #hasSystemFeature(String, int)} to check for a minimum version.
 *
 * @return Returns true if the devices supports the feature, else false.
 */
public abstract boolean hasSystemFeature(String name);

/**
 * Check whether the given feature name and version is one of the available
 * features as returned by {@link #getSystemAvailableFeatures()}. Since
 * features are defined to always be backwards compatible, this returns true
 * if the available feature version is greater than or equal to the
 * requested version.
 *
 * @return Returns true if the devices supports the feature, else false.
 */
public abstract boolean hasSystemFeature(String name, int version);

都是抽象方法，我们去PMS查找对应的实现

PackageManagerService.java