Spring加载bean配置文件的schemas文件（懒加载）

曾经终败给现在 2021-09-28 20:46 599阅读 0赞

最近在调试Spring源码时，看到加载Spring的xml文件-schema部分，

里面使用了懒加载的写法（PluggableSchemaResolver的getSchemaMappings方法）

为了能更好理解这个懒加载（里面获取到的Map<String,String>是单例的），我依葫芦画瓢，写了一个简化版的懒汉式单例（采用双重检查锁）。

下面是类以及单元测试

import java.util.HashMap;
    import java.util.Map;
    
    /**
     * Created by shucheng on 2019-5-31 上午 10:24
     */
    public class MySchema {
    
        public static final String DEFAULT_SCHEMA_MAPPINGS_LOCATION = "META-INF/spring.schemas";
    
        private final ClassLoader classLoader;
    
        private String schemaMappingsLocation;
    
        private Map<String, String> schemaMappings;
    
        public MySchema(ClassLoader classLoader) {
            this.classLoader = classLoader;
            this.schemaMappingsLocation = DEFAULT_SCHEMA_MAPPINGS_LOCATION;
        }
    
        public Map<String, String> getSchemaMappings() {
            Map<String, String> schemaMappings = this.schemaMappings;
            // 如果schemaMappings不为null，则直接返回数据，不需要重新创建对象
            if (schemaMappings == null) {
                // System.out.println("线程" + Thread.currentThread().getName() + "进入if判断");
                synchronized (this) {
                    // System.out.println("线程" + Thread.currentThread().getName() + "进入同步锁");
                    // System.out.println("线程" + Thread.currentThread().getName() + "\n" +
                            "schemaMappings=====" + (schemaMappings != null ? schemaMappings.hashCode() : null) + "\n" +
                            "this.schemaMappings=====" + (this.schemaMappings != null ? this.schemaMappings.hashCode() : null));
    
                    schemaMappings = this.schemaMappings;
                    if (schemaMappings == null) {
                        System.out.println("开始加载schemaMappingsLocation");
                        Map<String, String> map = new HashMap();
                        map.put("1", "张三");
                        map.put("2", "李四");
                        map.put("3", "王五");
                        map.put("4", "赵六");
                        map.put("5", "钱七");
    
                        schemaMappings = new HashMap<>();
                        schemaMappings.putAll(map);
                        System.out.println("schemaMappingsLocation加载中");
    
                        this.schemaMappings = schemaMappings;
                        System.out.println("schemaMappingsLocation加载完成");
                    }
                }
            }
            // System.out.println("线程" + Thread.currentThread().getName() + "======class为" + schemaMappings.getClass().hashCode() + "=====" + schemaMappings);
            return schemaMappings;
        }
    
        @Override
        public String toString() {
            return "MySchema{" +
                    "schemaMappings=" + getSchemaMappings() +
                    '}';
        }
    }

import org.junit.Test;
    
    import java.util.Map;
    
    /**
     * Created by shucheng on 2019-5-31 上午 10:23
     */
    public class MyTest {
    
        // 简单测试
        @Test
        public void test() {
            MySchema m = new MySchema(getClass().getClassLoader());
            System.out.println(m);
            // System.out.println(m.toString());
        }
    
        // 模拟多线程（3个线程）测试
        // 参考链接：https://www.jb51.net/article/130924.htm
        // 另外还参考了MyBatis3.5.1源码里的单元测试org.apache.ibatis.io.VFSTest#getInstanceShouldNotBeNullInMultiThreadEnv
        @Test
        public void testMultiThread() {
            final int threadCount = 20;
            Thread[] threads = new Thread[threadCount];
            InstanceGetterProcedure procedure = new InstanceGetterProcedure(new MySchema(getClass().getClassLoader()));
    
            for (int i = 0; i < threads.length; i++) {
                String threadName = "Thread##" + i;
                threads[i] = new Thread(procedure, threadName);
            }
    
            for (Thread thread : threads) {
                thread.start();
            }
        }
    
        private class InstanceGetterProcedure implements Runnable {
    
            private MySchema mySchema;
            Map<String, String> schemaMappings;
    
            public InstanceGetterProcedure() {
            }
    
            public InstanceGetterProcedure(MySchema mySchema) {
                this.mySchema = mySchema;
            }
    
            @Override
            public void run() {
                // 获取schemaMappings
                schemaMappings = mySchema.getSchemaMappings();
            }
        }
    }

这里我重点说下多线程的情况：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTA5OTk4MDk_size_16_color_FFFFFF_t_70][]

这个属性被赋过两次值，这点我开始很疑惑，一直想能否把第33行去掉。答案当然是不能去掉（因为去掉的话，多线程情况下有可能返回的不是单例） 根源就是getSchemaMappings方法加锁只是对部分代码加锁，第23行就没有加锁，完全有可能出现多个线程进来后，每个线程的schemaMappings属性都为null

这一点可以通过调用MyTest的testMultiThread单元测试方法来证实，多调用几次就会碰到我所说的情况，下图是我碰到的一次

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTA5OTk4MDk_size_16_color_FFFFFF_t_70 1][]

去看这个问题的起因：

我之所以会注意到这点，是因为我在用idea调试PluggableSchemaResolver中★★★位置（第93行）时，看到schemaMappings的值为null；但是当这个位置向下再执行一行代码后，鼠标放到schemaMappings上发现已经有结果了。

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTA5OTk4MDk_size_16_color_FFFFFF_t_70 2][]

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTA5OTk4MDk_size_16_color_FFFFFF_t_70 3][]

![20190531145846336.png][]

这种现象让我感到很奇怪，因为这行代码并没有直接操作schemaMappings属性，本身这个类也没有被代理，只是一个普通方法。

然后我在这个类当中继续搜索schemaMappings，发现有一个getSchemaMappings方法（里面直接操作了schemaMappings属性），而这个方法被两个地方（toString和resolveEntity，后面这个和我看到的现象无关，和实际解析xml时先加载schemas有关）调用了。这时我猜想debug时，idea会监控类以及其中变量，我想到现象是idea监控类的时候调用了toString方法（因为要在Variables显示类的有关信息）

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTA5OTk4MDk_size_16_color_FFFFFF_t_70 4][]

如果我直接运行单元测试，去掉所有断点，发现在★★★位置后初始化完schemaMappingsLocation变量后，直接在加上System.out.println("==========================" + schemaMappings);来运行，发现打印出来的是空的

这就证实了我前面猜想的debug过程中idea会调用监控类的toString是对的，所以我一开始看到的现象也就解释得通了

附PuggableSchemaResolver的部分源码：

public class PluggableSchemaResolver implements EntityResolver {
    
    	/**
    	 * The location of the file that defines schema mappings.
    	 * Can be present in multiple JAR files.
    	 *
    	 * 默认 {@link #schemaMappingsLocation} 地址
    	 */
    	public static final String DEFAULT_SCHEMA_MAPPINGS_LOCATION = "META-INF/spring.schemas";
    
    
    	private static final Log logger = LogFactory.getLog(PluggableSchemaResolver.class);
    
    	@Nullable
    	private final ClassLoader classLoader;
    
    	/**
    	 * Schema文件地址
    	 */
    	private final String schemaMappingsLocation;
    
    	/** Stores the mapping of schema URL -> local schema path. */
    	@Nullable
    	private volatile Map<String, String> schemaMappings; // namespaceURI与Schema文件地址的映射集合
    
    
    	/**
    	 * Loads the schema URL -> schema file location mappings using the default
    	 * mapping file pattern "META-INF/spring.schemas".
    	 * @param classLoader the ClassLoader to use for loading
    	 * (can be {@code null}) to use the default ClassLoader)
    	 * @see PropertiesLoaderUtils#loadAllProperties(String, ClassLoader)
    	 */
    	public PluggableSchemaResolver(@Nullable ClassLoader classLoader) {
    		this.classLoader = classLoader;
    		// ★★★
    		this.schemaMappingsLocation = DEFAULT_SCHEMA_MAPPINGS_LOCATION;
    		// System.out.println("==========================" + schemaMappings);
    	}
    
    	/**
    	 * Loads the schema URL -> schema file location mappings using the given
    	 * mapping file pattern.
    	 * @param classLoader the ClassLoader to use for loading
    	 * (can be {@code null}) to use the default ClassLoader)
    	 * @param schemaMappingsLocation the location of the file that defines schema mappings
    	 * (must not be empty)
    	 * @see PropertiesLoaderUtils#loadAllProperties(String, ClassLoader)
    	 */
    	public PluggableSchemaResolver(@Nullable ClassLoader classLoader, String schemaMappingsLocation) {
    		// 暂时用不到，这里省略具体内容
    	}
    
    
    	@Override
    	@Nullable
    	public InputSource resolveEntity(@Nullable String publicId, @Nullable String systemId) throws IOException {
    		// 暂时用不到，这里省略具体内容
    	}
    
    	/**
    	 * Load the specified schema mappings lazily.
    	 */
    	private Map<String, String> getSchemaMappings() {
    		Map<String, String> schemaMappings = this.schemaMappings;
    		if (schemaMappings == null) {
    			synchronized (this) {
    				schemaMappings = this.schemaMappings;
    				if (schemaMappings == null) {
    					// System.out.println("开始加载schemaMappingsLocation");
    					if (logger.isTraceEnabled()) {
    						logger.trace("Loading schema mappings from [" + this.schemaMappingsLocation + "]");
    					}
    					try {
    						Properties mappings =
    								PropertiesLoaderUtils.loadAllProperties(this.schemaMappingsLocation, this.classLoader);
    						if (logger.isTraceEnabled()) {
    							logger.trace("Loaded schema mappings: " + mappings);
    						}
    						schemaMappings = new ConcurrentHashMap<>(mappings.size());
    						CollectionUtils.mergePropertiesIntoMap(mappings, schemaMappings);
    						this.schemaMappings = schemaMappings;
    						// System.out.println("schemaMappingsLocation加载完成");
    					}
    					catch (IOException ex) {
    						throw new IllegalStateException(
    								"Unable to load schema mappings from location [" + this.schemaMappingsLocation + "]", ex);
    					}
    				}
    			}
    		}
    		return schemaMappings;
    	}
    
    
    	@Override
    	public String toString() {
    		return "EntityResolver using schema mappings " + getSchemaMappings();
    	}
    
    }

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[20190531145846336.png]: /images/20210923/a41fed56ce8e41f488a9371cc7b713c1.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTA5OTk4MDk_size_16_color_FFFFFF_t_70 4]: /images/20210923/d2d974e4a6894a578ff6d41c355f0590.png