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preface

  • In the process of reading the source code, you will often see the source code calledSystem.getProperty(...)Gets system properties.
  • In this article, I will lead you to understandSystem.getProperty(...)Source code, if you can help, please be sure to click like plus attention, this is really very important to me.

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1. Initialization

First, let’s look at where the system properties are initialized/loaded:

System.java

Private static Properties props; System Properties that cannot be modified private static Properties unchangeableProps; private static native String[] specialProperties(); (simplified) static {1, initialization is not allowed to modify the system properties unchangeableProps = initUnchangeableSystemProperties (); 2, all initialization system properties props = new PropertiesWithNonOverrideableDefaults (unchangeableProps); . } - > (1) has been simplified to private static Properties initUnchangeableSystemProperties () {VMRuntime runtime = VMRuntime. GetRuntime (); Properties p = new Properties(); P.ut ("java.boot.class.path", Runtime.bootclasspath ()); p.put("java.class.path", runtime.classPath()); 1.2 Call specialProperties() to load the property parsePropertyAssignments(p, specialProperties()); . } - > 2 Properties of defaults are not allowed to change the static final class PropertiesWithNonOverrideableDefaults extends the Properties { PropertiesWithNonOverrideableDefaults(Properties defaults) { super(defaults); } @Override public Object put(Object key, Object value) { if (defaults.containsKey(key)) { return defaults.get(key); } return super.put(key, value); } @Override public Object remove(Object key) { if (defaults.containsKey(key)) { return null; } return super.remove(key); }}Copy the code

The above code has been simplified and we focus on the following points:

1, construct the unchangeableProps property object, and initialize all system properties;

2. Construct the props property object and pass in unchangeableProps as the default property. Props is a subclass of PropertiesWithNonOverrideableDefaults, thus ensure the system property read-only cannot write;

1.2. Call specialProperties() to load the property, which is a native method.

2. SpecialProperties () source code analysis

In this section we will examine the source code for specialProperties(), using the Android platform as an example:

System.c

static jobjectArray System_specialProperties(JNIEnv* env, jclass ignored) { jclass stringClass = (*env)->FindClass(env, "java/lang/String"); jobjectArray result = (*env)->NewObjectArray(env, 4, stringClass, NULL); char path[PATH_MAX]; . Path) const char* LIBRARY_path = getenv("LD_LIBRARY_PATH"); #if defined(__ANDROID__) if (library_path == NULL) { android_get_LD_LIBRARY_PATH(path, sizeof(path)); library_path = path; } #endif if (library_path == NULL) { library_path = ""; } char* java_path = malloc(strlen("java.library.path=") + strlen(library_path) + 1); strcpy(java_path, "java.library.path="); strcat(java_path, library_path); jstring java_path_str = (*env)->NewStringUTF(env, java_path); free((void*)java_path); (*env)->SetObjectArrayElement(env, result, 3, java_path_str); return result; }Copy the code

Java.library. path = android_get_LD_LIBRARY_PATH(…); Obtaining path:

linker.cpp

void do_android_get_LD_LIBRARY_PATH(char* buffer, size_t buffer_size) { ... char* end = buffer; From default_LD_PATHS for (size_t I = 0; i < default_ld_paths.size(); ++i) { if (i > 0) *end++ = ':'; end = stpcpy(end, default_ld_paths[i].c_str()); } } } static void init_default_namespace_no_config(bool is_asan) { auto default_ld_paths = is_asan ? kAsanDefaultLdPaths : kDefaultLdPaths; . } #if defined(__LP64__) 64 bit system static const char* const kSystemLibDir = "/system/lib64"; static const char* const kVendorLibDir = "/vendor/lib64"; #else static const char* const kSystemLibDir = "/system/lib"; static const char* const kVendorLibDir = "/vendor/lib"; #endif static const char* const kDefaultLdPaths[] = { kSystemLibDir, kVendorLibDir, nullptr }; Static const char* const kAsanDefaultLdPaths[] = {omitted};Copy the code

The source code is not complicated. When we call System.getProperty(“java.library.path)”, we get two results:

  • For 64-bit systems, the return is", "the"/system/lib64 / vendor/lib64. ""
  • For 32-bit systems, the return is"/ system/lib", "/ vendor/lib"

When we look at these directories, we can see that there are indeed many.so libraries. When we call System.loadLibrary(…) When loading the dynamic library, the system’s built-in.so library is read/loaded from these paths.

3. Obtain and set system properties

3.1 Obtaining System Attributes

To get System properties, call System.getProperty(…). , the source is very simple:

System.java

public static String getProperty(String key) { checkKey(key); SecurityManager sm = getSecurityManager(); if (sm ! = null) { sm.checkPropertyAccess(key); } return props.getProperty(key); } -> public static SecurityManager getSecurityManager() {// no-op on android.return null; }Copy the code

3.2. Set system properties

To set System properties, call System.setProperty(…). , the source is very simple:

System.java

public static String setProperty(String key, String value) { checkKey(key); SecurityManager sm = getSecurityManager(); if (sm ! = null) { sm.checkPermission(new PropertyPermission(key, SecurityConstants.PROPERTY_WRITE_ACTION)); } return (String) props.setProperty(key, value); }Copy the code

Two points to note:

  • Built-in system attributes cannot be modified;
  • For custom attributes,SystemIt’s not persisted.

4. Common system attributes

Here are some common system attributes and their descriptions:

5. To summarize

  • 1, although system attributes are not the core knowledge, but understanding some key conclusions of this article is helpful to read more in-depth source code;
  • 2, the source of system attributes, part by JDK management, part by the specific runtime environment management (through native methodspecialProperties());
  • 3. Built-in system attributes are not allowed to be modified, and the modification of attributes is not stored persistently.

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